Descending the staircase in a baronial mansion, wandering through a Japanese garden, watching antique planes take off and land -- these are museum experiences that await you off the Mall.

With Smithsonian museums the ranking members of the local museum family, poor cousins of eccentric character are often neglected. But the smaller museums in Washington possess hidden treasures to enjoy at your leisure, away from the crowd.

More than cases of exhibits, these museums are special environments smelling of musk, rose petals or cinnamon. Friendly docents may greet you at the front door with their undivided attention -- and endless tales of former residents. The museums are small enough to do in an hour or so.

Comparing these museums -- places with charm and personality -- to the Smithsonian is like measuring a pastry shop against a gourmet supermarket. It just can't be done.

Here's a sampling of ten of them. THE RICH AND THE SUPER-RICH -- HILLWOOD, the home of the late Marjorie Merriweather Post, visitors rave over her Russian chalices, icons, kovshi, samovars, imperial porcelain and Faberge, and fuss over her French china. But it's the intimate personal details they want most. Somehow before the tour of the mansion is over, they have wrested from the guide the fact of Post's four husbands and that she probably could have bought and sold most of them. (By the way, "Mrs. Post," as she is reverently called by the guides, refers to her maiden name, her father being the cereal magnate C.W. Post.) Visitors also express great curiosity about Post's child by E.F. Hutton, actress Dina Merrill. "Ooh, is that Dina?" they ask as they pass her portrait. Visiting Hillwood automatically puts them on a first-name basis. Beyond family history (which includes many dogs interred in the pet cemetery), Hillwood touches on Russian and French history. One would do well to bone up in advance on czars and French monarchs. Portraits of Catherine the Great line the grand staircase, a roll-top desk made for Marie Antoinette sits in the 18th-century-style French drawing room, and there are in the collection countless gifts from one czar or czarina or another: You may want to keep track of them. The tour guides specialize in Russian or French history so they really rattle them off. And a second visit to Hillwood brings not only different tour guides but different narration, because there are just too many things. "The Russian collection is considered not only the biggest, but the most comprehensive outside the Soviet Union," says Anne Odom, public relations assistant at Hillwood. Purchased by Post in 1955, Hillwood took two years to remodel in order to house the collection that she acquired when her third husband, Joseph E. Davies, was ambassador to Russia in the late 1930s. She died in 1973, and Hillwood opened as a museum in 1977. It's a house museum, and as such nothing is labeled, which can be frustrating at first to one accustomed to the standard museum format. Though in the icon room Post installed a few pull-out trays with explanations for her guests, she designed the display cases so they would blend with the decor and daily life. Tours move along at a brisk clip, and another one is always bringing up the rear. A dour butler closes the double doors behind you, even before you can examine, say, the four china services Catherine the Great ordered for use at the annual dinners she gave her imperial knights. Of special note is the pavilion, where movies were shown (the help watched from their own balcony), and sometimes there were square dances. Attesting to Post's love of the dance, about 50 pairs of dancing shoes, identical except for color, can be found in a closet along with her square-dance outfits. Kept in much the same state as it was when Post lived here, the house has the wistful, yearning quality of a Romeo who has lost his Juliet. In the drawing room, adorned in wood paneling Post rescued from a French chateau being torn down, a single tea cup and saucer are poised next to an English sterling tea set ("One of of her many, I should say," offers the guide). The breakfast room is lit by a sparkling bay window by day and a chandelier by night ("It was supposed to have been in the bedroom of Catherine the Great. If only the chandelier could talk," says the guide). A fountain trickles, topped by a statue of Pan, and on a table set for four in Sevres' bleu celeste china, freshly cut snapdragons await no one's pleasure. A Hillwood tour should be done in two parts -- one day, the house, and another, the grounds, where a dacha (built over the swimming pool) houses a smaller Russian collection, another building holds American Indian artifacts, and a third, the C.W. Post wing, keeps the belongings of Marjorie Merriweather Post's father. This wing is like an attic of things not good enough to go into the big house: ponderously heavy Victorian furniture, copies of ornate Italian rocking chairs and so forth, and suits of armor looking like tin cans. Judging from this, Post didn't inherit her taste from her father. The grounds are best done on a fine day in late April or early May, when the azaleas are in bloom. But for the moment suffice roses, dahlias, orchids in the greenhouse and lavender in the herb garden, visited by orioles in this tamed corner of Rock Creek Park. Behind the house, blue-and-white lawn chairs -- unused, eerily gay -- dot the lawn. Nearby in the Japanese garden, among the wooden bridges, lanterns and waterlilies, a lightly tumbling waterfall whispers of a lost Eden. The words "You can't take it with you" come to mind. HILLWOOD -- 4155 Linnean Avenue NW. Tours, by reservation only, at 9, 10:30, 12 and 1:30. Closed Tuesday and Sunday. Admission $7. Grounds only, 11 to 3:30, no reservaions necessary, $2. Children under 12 not admitted. Call 686-5807. WILBUR WAS HERE -- Most museums commemorate events that happened somewhere else. But the COLLEGE PARK AIRPORT MUSEUM sits on the edge of what claims to be the world's oldest still-active airport. There, in 1909, Wilbur Wright instructed the first flying officers of the U.S. Army Signal Corps and a woman took a plane ride for the first time. Over the years it was understood the fliers were making history, and someone was always photographing them. Photos and film footage are part of the display, as are propellers, an instrument panel, a mail bag, a piece of the original wing material of the Wright plane and early charts advising to "Always look behind, on either side and in front before opening out your engine to take off. There may be another machine about to land in your way." The rumble of engines from the airfield makes it authentic. "A lot of people are starting to realize the significance of the airport," says Cathy Wallace, airport historian. "It's almost right up there with the Wright Brothers." This year, the 200th anniversary of flight in America and the 75th anniversary of this airport make it a good time to be there, sitting in the bleachers outside the museum and watching the single-engines take off and land. If you look closely, you'll see a few wooden propellers among the metal ones. And this weekend, the College Park Air Fair takes off Saturday, 10 to dark (followed by fireworks) and Sunday 10 to 6, at the airport. Look for antique planes flying in, military aircraft flying over, plane rides and a flying circus. It's free except for rides and refreshments. COLLEGE PARK AIRPORT -- 6709 Cpl. Scott Drive, College Park. Open Friday, Saturday and Sunday, noon to 4. Call (museum hours) 864-1530. GOT A TISSUE? -- From "the Old Red Brick," the original medical museum that was torn down on the Mall to make way for the Hirshhorn, the ARMED FORCES MEDICAL MUSEUM at Walter Reed still has the old faves: -- The hair ball in the shape of a stomach (what happens when a girl eats her hair for six years). -- The elephantiasis leg, dead-white in a jar. -- Selections of gallstones like matching pearls. -- Medals, buttons and a denture removed from windpipes. Take this museum as a warning. Preserved human tissues in a succession of glass cases run the gamut of bodily disrepair, from stab wounds to stomach ulcers. A coal miner's lung graphically illustrates black-lung disease. And color photos of gum disease are enough to make you floss on the spot. Distancing is achieved, however, with pure science. The Billings Collection, the largest collection of microscopes in the world, spans the centuries from the first compound microscope made around 1600 to scanning electron microscopes. Endless rows of plastic gewgaws prove to be an array of artificial heart valves. At the medical museum, you can compare your lifestyle with that of the "oldest man," a Colombian who lived to the ripe-old estimated age of 162. If that's not enough of a lesson, a push- button test by the door surveys your bad habits and rates your life-expectancy. Visitors tend to drive away from this museum slowly. But as personally fascinating as the place may be, your enthusiasm probably won't compare to that of Civil War soldiers, who used to visit to see their own amputated arms or legs. ARMED FORCES MEDICAL MUSEUM -- On the grounds of the Walter Reed Army Medical Center, 6825-16th Street NW. Open weekdays 10 to 5, Saturday and Sunday, noon to 5. Call 576-2418. THE BEER BARON -- The beer that made Washington famous? Not quite. The Heurich Brewery -- producer of Senate Beer and Old Georgetown Beer -- closed in 1956. But German brewmaster Christian Heurich left us the entrance to a time tunnel instead. The HEURICH MANSION, now headquarters of the COLUMBIA HISTORICAL SOCIETY, shows life as it was in the Gilded Age. It's like nothing else in Washington: elaborate plaster moldings, hand-stenciled walls and wallpaper, painted nymphs floating on the ceiling, 13 carved wooden fireplaces inset with marble, no two alike. Medallions line the foyer like misplaced door knockers and climb the walls to the second floor. Medallions were trendy in German castles. Heurich had the house built in the 1890s, and not only is the interior basically unchanged, most of the Victorian furnishings are original. Heurich imported many of the heavy home furnishings from Germany, which he left in 1866 when he was 24. He lived to 102. "His life offers some interesting clues," says Elizabeth Miller, curator of research collections for the historical society. "He didn't eat red meat for the last 40 years of his life. He took regular exercise, didn't retire, had a comfortable lifestyle, too, of course. Didn't smoke." He did drink the beer he brewed, though, and the rathskeller/breakfast room in the basement is evidence of that. Canvas wall coverings extol in German the virtues of brew. To continue the motif, a whimsical hat rack consists of crossed spears and antlers. Heurich's sense of humor leaned toward practical jokes as well. According to one family legend, he attached a humidor to a music box, so that he could tell when a particular preacher was helping himself to a cigar when no one was looking. While gilt French rocking chairs and white mahogany decorate the parlor, and a musicians' balcony and a gilt Steinway piano embellish the music room, the bathrooms are the most popular stops on the tour of this baronial mansion. They're very modern-looking for 1890, with their predominantly white color scheme and use of tile. Close to Dupont Circle, the quiet garden behind the house attracts office workers like bees, luring them from surrounding buildings to sit in the grass and eat bag lunches. Inside and out, the house is a rare Victorian gem. "There are a few house museums that tend to be earlier -- Decatur or Octagon House," notes Miller, "or later -- like Anderson House or Wilson House. But this is really it for depicting life in the late 19th century in a city, in Washington certainly." CHRISTIAN HEURICH MANSION/COLUMBIA HISTORICAL SOCIETY -- 1307 New Hampshire Avenue NW. Open Friday and Saturday. Tours given every half-hour starting at noon; last tour is at 3:30. Donation, $1. THE HACIENDA ON 18TH STREET -- The MUSEUM OF MODERN ART OF LATIN AMERICA is a misnomer. There really isn't any such thing as Latin American art -- any more than there is "women's art." What can be seen here is a diverse collection from different countries, cultures and traditions. Most unusual perhaps are the works of certain lyrical abstractionists who are Brazilians of Japanese ancestry. The two cultures blend in their work: Vivid, electric colors of Latin America combine with the composition and calligraphy of the Orient. Abstract, highly textured, almost poetic, the paintings contrast with primitives, also represented here, and their compulsion for detail. No less exciting, a Jamaican street filled with reggae dancers has been captured in a painting by a leader of the Rastafarian sect. The Museum of Modern Art of Latin America is not exactly a museum, either, but inviting galleries to ramble through, pausing awhile on soft benches. Behind the Organization of American States, the hacienda where the museum dwells was built by Andrew Carnegie in 1912. Blue tile and a terra cotta fresco adorn the rear portico. This overlooks a lilypond in the Aztec Garden, where presides a statue of Xochipilli, the "flower prince." The blue and terra cotta colors are used in lighter tones in the galleries, which were spruced up last year thanks to a grant from the National Endowment for the Humanities. Handouts on the artists and their styles have been made available in each gallery. However, you may know more about these Latin American artists than you think you do. Among the artists represented in the collection, Eduardo Villamizar sculpted "Tribute to John Kennedy," on the grounds of the Kennedy Center; Alejandro Otero produced "Solar Delta," just west of the Air and Space Museum, and Candido Portinari painted the murals in the Library of Congress' Hispanic Division. You may not see the works of this trio at the Museum of Modern Art of Latin America the first time you visit. The collection owns more than 600 works by 250 artists, but can only show 120 at a time, so the paintings are rotated every six months. MUSEUM OF MODERN ART OF LATIN AMERICA -- 201 18th Street NW. Tuesday through Saturday, 10 to 5. Call 789-6016. MEIGS' OLD RED BARN -- Step inside the NATIONAL BUILDING MUSEUM, and you are standing in the middle of its sole exhibit -- the Pension Building. They are one and the same. With the whirring of drills and the pounding of hammers, touring the old edifice while it's under renovation is like walking through a working model of a human heart. The concerted effort of the workmen here is directed toward a goal Ronald Reagan and Walter Mondale can appreciate -- Presidential Inauguration, 1985. The building is slated to be finished then, to continue a tradition of inaugural galas there. In the meantime, students of architecture can take a unique look at renovation in progress -- tours of the 1880s building are given twice a week. It's also a chance to witness the birth of a museum: Mandated by Congress in 1980 and dedicated to the building arts, the museum next year plans exhibits on architectural drawings and models and on the building's architect, General Montgomery C. Meigs. The volunteer guide apologizes for the condition of the place. But there's something appealing about its barebones look: the enormous Great Hall the size of a football field, pinned to the sky by eight Corinthian columns, and surrounded by arcaded galleries that held offices behind their arches. The architect copied the plan from Italian Renaissance palaces. He originally wanted the ceiling painted sky blue, and so it has just been repainted that color. A false Siena marble finish will return the columns to their original faux marble; for now, workmen can be seen carefully scraping away previous paint layers at the base. The massive, hollow columns are each made of 55,000 bricks, part of the architect's plan to fireproof the building. In addition, Meigs wanted "no dark corners in the building and no ill ventilated rooms," and his master plan took care of this, including what looks like three missing bricks under each window -- three holes to let in fresh air. From the outside, the most prominent feature of this red brick building is the terra cotta frieze that wraps around it; Casper Buberl designed this military parade that marches back on to itself, and signed his name under a few horses. At one time, the building was called "Meigs' Old Red Barn." It has managed to survive threats to tear it down. And now that it's being cleaned from the outside in, the original exterior color is emerging from years of grime -- revealing the true meaning of brick red. NATIONAL BUILDING MUSEUM -- Entrance on F Street NW between Fourth and Fifth streets. Tours: Tuesday at 11 and Thursday at 12:15. Some twilight tours at 5:45 (call for specific dates). Reservations required for all tours. Call 272-2448. REVOLUTIONARY IDEAS -- Excavated in archeological digs, shards are often all that remain from a beautiful porcelain bowl or plate made in Revolutionary days. But the china at the MUSEUM is still in fine condition, cherished and handed down through generations: Chinese export porcelain, Wedgwood creamware, redware, Jasperware, Delft. And seeing them with American-made stoneware and glassware, one begins to get an idea of what life was like -- for the elite, that is. "Most museums have that problem," says associate curator Susanne Dawson, "because what is saved are the objects that are elegant and valuable, and the everyday, common things are discardd." The museum tries to cut off the collection at 1830, but a few items from closer to 1850 have slipped in. The museum is housed in the building on D Street that links DAR Constitution Hall to the old Memorial Continental Hall facing 17th Street. The devoted daughters of the DAR have been generous. They've filled their museum and period rooms with treasures: among them, silver spoons made by Paul Revere and a tea chest believed to have gotten wet at the Boston Tea Party. In fact, as you pass from the museum to Memorial Continental Hall, you begin to notice that everything has been donated, brick by brick. The bricks tell the tale. To wit: "This elevator was given in memory of Josiah Bartlett, signer of the Declaration of Independence, with his wife Mary Bartlett, by one of their descendants." In Memorial Continental Hall, a guide will show you the 33 period rooms decorated by 33 states. Used to be they were all quite formal rooms -- Chippendale, high ceilings, chandeliers -- but some have been reesigned to more earthy, distinctive styles. Oklahoma's is a kitchen smelling of cinnamon, with an exposed beam ceiling hovering over pewter, candlesticks, apple press and spinning wheel. With things made or used in Texas, the room of the Lone Star State is an 1860-ish bedchamber with wardrobe, trundlebed and washstand. New Hampshire's room stands out. Dozens of dolls eye you from their little chairs as you walk in. This is the children's attic, home of the 19th-century toy collection, tracing the evolution of dolls from wood-carved and rag to bisque and papier mache. A devotion to the past here rubs off on visitors. As for the present, this Wednesday, an antique quilt show is opening -- "Sleeping Under the Stars: Star Motifs on Traditional Quilts." DAR MUSEUM -- 1776 D Street NW. Hours, Monday through Friday, 8:30 to 4, Sunday, 1 to 5. Closed Saturday. Call 628-1776. HOUSE AT LOGAN CIRCLE -- Civil rights leader and educator Mary McLeod Bethune made a house near Logan Circle her home and the headquarters of the National Council of Negro Women. On Vermont Avenue, it's now the BETHUNE MUSEUM AND ARCHIVES. With most of Bethune's memorabilia lodged in her former Daytona Beach home, the museum instead promotes the contributions of all black American women. In addition, for those Washingtonians who've never visited a Logan Circle house, this is a charming example of a 19th- century Victorian townhouse -- the notched trim outside, and inside, the marble fireplace, crystal chandelier and heavy dark wood doors. No longer furnished like a home, it provides a backdrop for the changing exhibits. At the moment, one on the pioneers of black beauty culture tells how Sarah Breedlove, better known as Madam C.J. Walker, whose parents had been slaves, invented a hair-growing formula and became a millionaire around the turn of the century. This contrasts with a show of watercolors by Lois Mailou Jones, whose work has spanned more than 50 years. Her lovely scenes of Italy and Monte Carlo stand out even more against the brick-in-relief of the building next door, seen through the long Victorian windows. BETHUNE MUSEUM -- 1318 Vermont Avenue NW. Monday through Friday, 10 to 4:30. Closed Saturday. Tours Sunday by appointment only. First and third Sundays of the month, concert or writer-readings at 3. Call 332- 1233. AH, BLISS -- DUMBARTON OAKS is the most beautiful of the small museums, with a wing designed to house the Robert Woods Bliss collection of pre-Columbian art. Central to a series of acoustically perfect rotundas, a fountain perpetually thrashes. Cool. Quiet. Except for the crunch, crunch of rubber-soled shoes on polished wood and marble floors. It is said that a word whispered in just the right part of one of these circular pavilions echoes clearly throughout the chamber. This is true. A couple's furtive conversation was recently overheard from across the room. She: "I forgot! If you want your suit, we have to leave at 4." He: "That's okay. I'll get it tomorrow." Eavesdropping has to be more revealing on the next visit. But the man has a point, in that it is hard to leave Dumbarton Oaks. There's a European air to the place (and many of the guards have wonderful foreign accents). Because it isn't crowded and visitors move at leisurely pace, here, too, is a rare feeling of being left alone with the art objects. The main house was built in 1800, but Robert and Mildred Bliss made many enlargements. Their similar taste in art has been attributed to the fact that, when Robert and Mildred were teenagers, their parents, both widowed, married each other. The children were each independently wealthy. During his career with the foreign service, Robert Bliss was ambassador to Argentina. After her husband died, Mildred Bliss added the wing for the Garden Library that holds rare gardening books as well as a few impressionist paintings. Beyond a courtyard mosaic pool, where an image of the Titan goddess Tethys has been swimming since the sixth century, is the Byzantine collection, emphasizing the minor arts of that culture. Here can be seen sumptuous jewelry with religious motif, gold crosses and ecclesiastical silver from the sixth century and carved ivory icons from the 10th. At the moment, "The Plan of St. Gall" has taken over the Music Room. El Greco's "The Visitation" goes temporarily unnoticed next to this 42-panel exploration of a plan for a ninth- century Benedictine monastery. At Dumbarton Oaks is a facsimile reproduction of the Plan of St. Gall, which has been called a "Swiss national treasure." The plan has remained in its original home, the former monastery of St. Gall, for the past 1,200 years. It plots out both secular and ecclesiastical buildings -- from the sacristy to the medicinal herb garden (specific down to the herbs), from the house for bloodletting to the house of the fowlkeepers, from the granary to the monk's bake and brew house (a great name for a restaurant). The show will be uthrough September 16. A peacefulness comes over you at Dumbarton Oaks. To hold on to the feeling when you leave the museum, walk around the corner and let yourself into the grounds. It's another private world, where the musk of boxwood is pervasive, the summer garden is in bloom, and the annual chrysanthemum explosion is imminent. DUMBARTON OAKS -- 1703-32nd Street NW. Museum hours, Tuesday through Sunday, 2 to 5. Garden library, Saturday and Sunday only, 2 to 5. Grounds, daily 2 to 6 (grounds admission, $1). THE PRESIDENTS' NEIGHBOR -- The life and death of Stephen Decatur make a fascinating Washington story. A hero of the Barbary Coast and the War of 1812, Decatur hired architect Benjamin Latrobe to design a house on Lafayette Square where he lived for 14 months with his wife Susan. Sought by Washington society, the naval hero and his beautiful wife had their social whirl cut short by a duel in March, 1820. Commodore James Barron, who believed that Decatur had ruined his naval career, challenged him in Bladensburg, Maryland. "The idea of the duel was so stupid," says Jean Dempsey as she gives a tour of DECATUR HOUSE. And Susan Decatur didn't know about the duel until her mortally wounded husband returned home that night. Since his death there have always been rumors, persistent as gnats, that his ghost wanders the house. As a widow Susan Decatur feared poverty, so over time sold off the couple's possessions. Now the National Trust for Historic Preservation has been buying them back from their owners, and 22 of the acquisitions of the past 10 years are on display at Decatur House in a self-guided tour, until September 30. Among them is an 1815 engraving that depicts Decatur's capture of the British frigate, the Macedonian. His strategies are still taught at the Naval Academy, says Dempsey. "But we don't talk about the fact that he was blockaded in New York harbor for about six months," she adds. The Decatur House is divided into two periods: the symmetrical federalist style downstairs, as it was restored to Latrobe's original plan by a later occupant, Washington hostess Marie Beale, wife of diplomat Truxton Beale; and Victorian upstairs, with windows lowered and handsome parquetry floors installed by the Beale family. "Every president has been here since Madison," says Dempsey, as she names the famous visitors. Occupants have included secretaries of state Henry Clay and Martin Van Buren, and John Gadsby, the local tavern owner. Union officers used it as headquarters in the Civil War. It was later bought by Mary Edwards Beale, wife of the adventurer General Edward Fitzgerald Beale, who led an American expedition to the Colorado River using camels. (For the incredulous, this feat is depicted in paintings that hang in the house.) Decatur House is full of antiques, including two dinner sets of Chinese export porcelain and a magnificent Hepplewhite sidebar. Of special interest are two old pistols, from the historic duel. The house commands a magical view of Lafayette Square: Where once Union troops encamped, now protesters set up their tents. Decatur House watches from its vantage point on the edge of history. DECATUR HOUSE -- 748 Jackson Place NW. Tuesday through Friday 10 to 2, Saturday and Sunday noon to 4. Admission, $2 for adults, $1 for senior citizens and students. Call 673-4030.