BY THE BOOK Take-One Tours, compiled by Don't Tear It Down Inc., tell you what to look at while riding the G2 between Howard and Georgetown universities, and the S2 and S4, along 16th Street NW. Booklets are supposed to be aboard the buses but usually aren't. Get them before you go by sending a stamped, self-addressed envelope to Don't Tear It Down at 1346 Connecticut Avenue, Suite 202, Washington 20036.

It's a sunny weekend morning, for a change, inspiring thoughts of how it would be at the beach - followed by thoughts of how it will be at the gas station, if you can find one open.

Never mind: While others queue up to fight the Bay Bridge traffic, just walk to the nearest bus stop. There'll be no line, no crowds. If it's Saturday, there may be a few shoppers; if Sunday, some people on their way to church. None need know that you're off to see what's at the end of the line, if there's any there there, or along the way. If you want to blaze your own trail, go down to Metro headquarters (600 Fifth Street NW) and get a batch of schedules; if you have a destination in mind, call 637-2437, where a voice will tell you how to get to wherever-it-is. But if you want to ease ease into adventure, to learn about the terrain first, read on. Here are some tried-and-true trips with some highly arbitary ratings.

ROUTE 81: THE PILGRIMS' WAY * * * Go on a Sunday and you'll feel that you've been to church - because you will have been. You'll also have seen a peaceful and pretty part of the city that's a little off the beaten path. It helps if you like ecclesiastical architecture.

The 81 ends up in the Fort Totten Metro parking lot, surrounded by apartment houses, but its spirtural destination is Brookland, once Irish and still full of Catholic institutions.

On the way, the bus calls at the Kennedy Center and winds through downtown and Chinatown. We got on at North Capitol and Massachusetts, next to the main Post Office and block from Union Station. On our left was the massive redbrick Government Printing Office, and across the street, on its Art Deco warehouse, reliefs panels show government workers printing everything from tax forms to tips on canning asparagus. Farther up North Capitol, note the Washington Coal Company, on your right, with silver masques smiling from each side of the building, and, on your left, Engine Company No. 12, an 1897 firehouse with an ornate, Dutch-style roof.

Just after you pass McMillan Reservoir, the bus turn onto Michigan Avenue and enters a sort of mini-Rome. First stop is Trinity College, where the Catholic elite traditionally sent its daughters. Behind its gray stone walls glows a gem of an art collection in the O'Connor Art Gallery; among its hundreds of 16th-to 20th-century paintings are miniatures of Old Masters in European collections.

Around the corner is the second-largest Catholic church in the world, the National Shrine of the Immaculate Conception, which some irreverent souls say was made to be covered with volcanic ash and discovered 4,000 years hence. A tour leaves the vistors center half-hour on weekends, hourly on weekdays.

Across the campus of Catholic University, next to the Brookland Metro, is Bellair, an 1830s Greek Revival mansion. Once slated to be demolished to make an even bigger parking lot, the vacant, rundown mansion may be saved in a complicated property swap between Metro and the D. C. government. Walk up the circular drive to the columned entrance portico, and check to see if there are any figs in the miniature orchard. Bellaire, which included several hundred acres, became one of Washington's first subdivisions - Brookland. The B&O railroad, whose tracks were laid in 1837 very close to the mansion, made the subdivision accessible to commuters.

We got off the bus again at the corner of 13th and Quincy Streets NE, practically in front of an ecumenical establishment that boasted both Chinese and American food - Edgies Carryout and the Lam Sun Restaurant - but were told that only the American food can be consumed on the premises.

So we carried our Chinese food up hilly Quincy Street and onto the grounds of the Franciscan monastery, where we had an unplanned picnic. (I'm not sure whether picnicking on the monastery grounds is officially sanctioned, but the brown-robed frairs who walked past us just smiled.)

The monastery grounds, open daily from 6 a. m. to 9 p. m., are like a hill town in Tuscany. The yellow-ochre, Byzantine-style church with its red tile roof, its columned portico and formal gardens, was built in 1988 to house replias of many of the famous Catholic shrines of the world. There aretours of the shrines, but for most visitors the highlight is the descent into the dark, narrow catacombs. Replicas of the underground haunts where persecuted Christains used to worship in secret, the catacombs occasionally open on mini-shrines holding relics - bones or other artifacts of saints. Outside, scattered around the grounds and grottos of cement shaped to imitate rock, are reproductions of such famous pilgrimage sites as Lourdes, Mount Olivet and the tomb of the Blessed Virgin.

No. N4: ECHO OF THE PAST. * * * The only thing wrong with this scenic route is that you'll wish it were 20 years ago and you could have taken a trolley to Glen Echo Park when it really was an amusement park. But attractions along Masschusetta Avenue, where the trolley didn't go, almost make up for it.

The N4 starts at Farragut Square, circles Dupont Circle, and then heads out along the city's longest and grandest boulevard - Massachusetts Avenue. Glance quickly at 2020 Massachusetts, formerly the mansion where Evelyn Walsh McLean used to flash her Hope Diamond. On your right, at 2021, stands the ritziest rooming house in town, the Argyle Guest House, set up in the '30s when hard time forced one Captain Miller and his wife to take paying guests into their mansion.Across the street, at 2118, is Anderson House, the museum and headquarters of the Society of the Cincinnati. Open from 2 to 4 every afternoon but Monday, it's worth a visit if you're interested in Revolutionary War history or French, English and Oriental furniture and antiques.

You can get off here, visit the museum and continue on foot till you get tired; on foot, you'll be able to read the names on the brass plates of each embassy so you won't have to guess their names from their flags.

The foreign-looking building at 2551 Masssachusetts isn't an embassy but the Islamic Center, largest mosque in the country, rich in rugs and mosaics. Visitors are welcome from 10 to 4 daily, except from 11:30 to 2:30 Fridays, when a prayer service is held.

As you pass the British embassy, on your left, wave to the bronze Sir Winston Churchill. He'll return the greeting with his V. In his other hand he holds a cane and a cigar that aroused some controversy among members of the English Speaking Union, which commissioned the work; the majority chose to leave it. One foot of the statue stands on British embassy turf, the other on American soil.

Just up the hill is a pleasant old home now occupied by Vice President Mondale and his family. This is not open to the public but the U. S. Navy Observatory, on the same grounds, is. Tour it Monday through Friday at 12:30 and 2 and see the clocks the Navy sets its watches by. During the evening tours, which you can get in on by writing to the Superintendent, you can peek through the Navy's gaint telescope.

At the crest of the hill, at the corner of Wiscousin Avenue, stands the Washington Cathedral complex. Tours of the 14th-century-style cathedral, with all its crypts, chapels and stained-glass windows - one contains a piece of moonrock - are given every day. If you're lucky, you may see a sculptor or stonecutter at work on the still-unfinished cathedral.

If you've stayed on the bus, or paid another fare you'll be passing venerable old apartment buildings, winding around Ward and Westmoreland circles and crossing into Maryland. When you reach the end of the line, you're about a block form Glen Echo Park. The trolleys ran smack up to the entrance, but they've been gone since the early '60s and the park has a ghostly air now.

"This is where the laughing lady used to be, remember?" said a visitor, walking past the shooting gallery toward the platform where the cars of the Cuddle-Up once held sway. The Crystal Pool stands empty, and no music flows from the boarded-up Spanish Ballroom - only the rhythm of classes in silversmithing and jewelry-making. The National Park Service, which now runs Glen Echo, has filled the once-frivolous park with more purposeful activities, mainly crafts classes. One reminder of the past that's still in working order is the 50-year-old carousel, with hand-carved animals and a real calliope. It's open summer weekends.

There's nothing much to eat in the park, but just outside the gates is Trav's, a downhome, family-type roadhouse with hamburgers like mother used to make and wooden booths with generations of initials carved in them. On the other side of the park, across a vast parking lot, lies the Clara Barton House, also operated by the Park Service. The rambling frame house, furnished in comfortable Victorian style, was built with boards salvaged from Red Cross shelters used during the Johnstown flood.

No. 40: LA CUARENTA, * * * This route passed through a mini-fiesta at 18th Street and Columbia Road. If you like food and street scenes better than buildings, this is the one for you.

A good place to get on - If you want to savor the historical significance of the 40 - is at the old trolley barn at 15th and East Capitol, which is to be restored as part of a luxury housing complex. From here the bus follows the route of the trolleys that once took passengers across town to Mount Pleasant.

As the bus loops around Lincoln Park, look to your left at a statue of black educator Mary McLeod Bethune, founder of the National Council of Negro Women. In the center of the park stands the Emancipation Memorial. The city's first monument to Lincoln, it was erected just after the Civil War and paid for by freed slaves.

Passing between the two Senate Office Buildings, the bus edges up to the National Visitors Center - a great place to visit if your kids have to go to the bathroom. Six days a week, the 40 evicts all passengers here, leaving them to make their way via Metrorail to Dupont Circle where they can pick up another 40. On Sundays, however, you can stay on the bus and keep your head above ground.

Along E Street, the bus passes nice old government buildings like the U.S. Tariff Commission building and ugly new ones like the F Street, it passes the soon-to-be revived Wellard Hotel, then turns up 14th Street to the adult-movie-and-bookstore strip. Then up Connecticut Avenue to Adams-Morgan.

You can go on to Mount Pleasant, where many old homes are being restored, but if you're in a festive mood, get off at 18th and Columbia. You'll be in the heart of one of city's liveliest ethnic communities, where Spanish music blares from record shops and vendors hawk bilingually. On a Sunday, you're likely to catch a small local band rehearsing in a mini-park. Families will be strolling, window-shopping at the Cordova Jewelry Shop, inspecting embroidered blouses from El Salvador, or munching fried donuts at the Churreria Madrid. If you're hungry, you have a global choice. 'There's Avignone Freres, a French tearoom; Angelo's Seafood Carryout, home of the Cuban sandwich; Carlos Gardel, boasting popular Argentina cuisine. If you're less adverturous but still want to get into the spirit of things, go to the McDonald's at 18th and Columbia Road, where the menu is in two languages. Order a grands Mac and papas fritas . For ambience, carry your food two blocks down Adams Mill Road to the Community Unity Park, a tract the people of Adams-Morgan have wrested from developers and use as a playground, garden plot and field for frequent games of beisbol.

OVER THE RIVER, UP THE HILL * * * Discover such neighborhoods as Le Droit Park and Uniontown and tour six (count 'em, six) museums, all of them free.

From 18th and Columbia - across from the intercultural McDonald's - you can catch the 94, which says it's going to Stanton Road but goes to a lot of other places besides. Following Florida Avenue through the still-unrewewed Shaw Urban Renewal Area, the bus skirts Le Droit, Park. Unfortunately, you can't see the grand, century-old homes and tree-lined streets from the bus, but you should consider getting off at Fourth and Florida for a walking tour of the neighborhood. About 50 of the large detached houses of the subdivision built on land purchased form Howard University remain. Though the streets now bear dull alphabetical designations, above the signs are some that tell of streets that used to be Juniper and Spruce and Elm.

Back on the bus, you'll keep coming across reasons to get off again. At Fourth and Florida NE, there's the Farmers Market where you can browse among the fresh produce and meat or select Italian delicacies or even eat a good steak dinner. Next to the market is Gallaudet College, world's only accredited liberal arts college for the deaf. The campus, known as Kendall Green, was laid out in the 1860s by landscape architect Frederick Law Olmstead, father of New York's Central Park. In the old chapel, which is now an exhibit center, a multi-media presentation called "the Look of Sound" shows visitors about hearing, deafness and programs to help deaf people.

Barely past the Gallaudet entrance, the bus turns up Eighth Street and journeys across Capitol Hill. Just south of Pennsylvania Avenue, Eighth Street becomes Barracks Row, named for the Marine Barracks across the street, a spruced-up area of antique shops, a tile shop, a basket shop, a fun Mexican restaurant and a fancy Swiss one.

Jefferson reportedly chose this site for the Marine Barracks partly because it was convenient to the Washington Navy Yard, just down the street. Get off the bus and walk through the yard to get a map at the Old Commandant's Office, with its wrap-around porches and widow's walk, which now serves as a visitors center. There's a combat art gallery, a Marine Corps Museum and a 19th-century forge shop recently turned into an attractive chapel. Kids love the Navy Memorial Museum, where you can go inside a submarine and look at the parking lot outside through a real periscope. Both in the museum and in Willard Park just outside, kids can climb on cannons, guns and rockets. On your way out through the main gate, look to your right at the present commandant's house. The tour booklet won't tell you this, but it's supposed to be haunted by the first commandant, Thomas Tingey. In any case, Tingey was so attached to the house that he left it to his descendants in his will. The Navy, however, intervened.

Outside the gate, another 94 will take you across the Anacostia River. When you see a huge dining-room chair - an advertisement a furniture store - get off and walk to the Anacostia Neighborhood Museum, at 2405 Martin Luther King Jr, Avenue. Open Monday through Friday 10 to 6, and weekends, 1 to 6, the Smithsonian affiliate has exhibits on the black heritage.

At the Shabazz Bakery next to the museum we bought some carrot cake to go and then, lest we get too healthy, got some fast hamburgers at a chain a few doors down. We carried our lunch east on W Street to 1411 and picnicked on the lawn of Cedar Hill, home of black abolitionist and Lincoln adviser Frederick Douglass, the first black to move into this suburb in 1877. A big, friendly dog joined us on the lawn.

"Is that Frederick Douglass' dog?" asked a child. It turned out to be the caretaker's dog, a relentless fetcher of sticks, and he added to the lived-in look of the house - as did the comfortable furnishings, many of which are original. Free guided tours are conducted Monday through Friday 9 to 4, and weekends 10 to 5.

To catch the flavor of old Anacostia, walk around the block bounded by 13th, 14th U and V Streets. If you get tired, stop to rest on a bench is a grassy park that serves as a sort of village green.

No. 11E: NEVER ON SUNDAY. * * * You may have been to Mount Vernon before, but this way getting there is fun.

Probably George and Martha Washington prayed in seclusion all day Sunday. In any case, they didn't travel between Washington and Mount Vernon by bus, because the 11E doesn't run on Sunday. It can, however, make a pleasnat Saturday excursion.

If you just want to go to Mount Vernon, take the 11E from 11th and Pennsylvania. If you want a more electric experience, take Matrorail to National Airport, making sure you pick up a transfer at the station where you board the train. If you have kids or secretly get a thrill out of watching planes take off and land yourself, splurge on a 10-cent ticket to the observation deck, off the airport's main lobby. (Inflation has not hit the observation deck because, airport officials confess, it would cost too much to alter the turnstiles). If you make arrangements in advance (by calling 557-2648), you can also tour the weather service and find out how they find out how fast the wind is blowing.

Then it's back to ground transportation, any 11 bus. Get off the bus at the corner of King and Washington Streets in Alexandria's Old Town. You might want to visit the crafts people of the Torpedo Factory, or browse your way through Dockside Sales. If you have kids along, let them ring the bells on the old engines at the Friendship Fire Company (107 South Alfred Street, Tuesday through Saturday 10 to 4) or see the Stabler-Leadbeater Apothecary Museum (107 South Fairfax Street, open Monday through Saturday 10 to 5). The Alexandria Tourist Council, in a restored 18th-century house at 221 King Street, will provide maps to show you where you want to go. Make your last stop Gadsby's Tavern (138 North Royal Street), where George Washington always sipped a few madeiras while waiting for the 11E to Mount Vernon.

After Old Town, the bus calls at large apartment complexes, then skirts the bike path that follows the river to Mount Vernon. The bus goes inland slightly, dropping you across the road from the mansion - or, rather, from the complex of concessions outside its gates. Once you get away from all that, however, you can get the feeling of and 18th-century working plantation. My almost-six-year-old was especially taken with the bed where Washington died, and most kids like the smokehouse, the kitchen that's not attached to the house, the wash house and the maze of garden paths lined with aromatic boxwoods. If you happen to be leaving Mount Vernon at just the right time - at 12:15 or at 5 - you can take the Washington Boat Line back to Southwest Washington. It's more expensive than the bus - $3 one way for adults and $2 for children - but probably more authentic. Some historians believe that George and Martha Washington usually took the boat - instead of the bus - when they traveled from Mount Vernon to Washington. CAPTION: Picture, Cover design by Terry Dale.