MORE THAN HALF the pleasure of exploring Washington is encountering the unexpected in well-known places. And it only takes half a day to discover some new dimensions and then savor them over a tasty lunch or coffee break. Visitors and even longtime area residents may want to try some new approaches to the following: A PORTRAIT IS A SILHOUETTE IS A LIFE CAST

"I never went in for family albums," said a relative of mine before his reluctant first visit to the National Portrait Gallery. Several hours later he emerged, impressed and enthusiastic. The "Hall of Presidents," top priority for many visitors, was merely the expected for him. What he wasn't prepared for were three- dimensional representations such as the terra cotta sculpture of Gertrude Stein by Jo Davidson. It's so effectively displayed and lighted in an alcove on the ground floor that every passerby seems to stop and admire it.

Equally compelling are the life casts of Lincoln's face and hands exhibited on the second floor. One set was done by Leonard Volk in 1860; the other by Clark Mills in 1865. "Nowhere is the toll the Civil War took on Abraham Lincoln more evident than in a comparison of the life masks," the Gallery invites the viewer to observe. The changes are, indeed, deeply moving.

Mathew Brady's box of glass negatives is in the Meserve Gallery next to the life casts. One of these negatives shows an image of Lincoln seated. The reality of immortality captured on a bit of treated glass is staggering.

Still another unexpected medium of protraiture is tucked among the paintings in a gallery devoted to westward expansion -- a tiny silhouette of Meriwether Lewis, signed "Dolley," and attributed to Mrs. Madison. On the first floor is a collection of silhouettes by Auguste Edouart, displayed in a Victorian studio setting. The real finraits is one of a 19th-century Chinese gentleman whose traditional queue almost reaches his ankles. In Chinese characters he wrote this description of himself: "From Pekin/ Arriving Canton Province / and Macao / Mr. Chin Sung / Portrait / Always chatting / with his friends / and looking at the moon / recites a poem."

A visit to the Gallery is even more of a treat with a lunch or tea in Patent Pending, the museum's self-service cafe (open 11 to 3:30 weekdays, 11:30 to 4 weekends). Handsome framed posters of past exhibits line the walls and picture windows frame an outdoor courtyard sheltered by two giant elms. The light fare includes soup, salads, croissant sandwiches and desserts. A "Walt Whitman" turkey salad sandwich costs $3.25. Beer and wine are available. NATIONAL PORTRAIT GALLERY

Old Patent Office Building, Eighth and F streets NW. Open 10 to 5:30 daily. Metered street parking; Gallery Place stop on the Metro Red Line; Metro Center stop at 11th and G streets on the Red, Blue and Orange Lines.

MILE ZERO-PLUS -- Bring your camera and wear comfortable shoes on this outing, which has nothing to do with boating. Enthusiasts of the C&O Canal know that the Canal began slightly to the right of the river side of the Watergate Complex. Now's your chance to see the modest but historically significant post marked "Mile O" on a bit of land beyond the tidelock just downstream from Thompson Boat Center.

But what Canal fans may not know is that the riverbank in front of the entrance to the boat center is one of the most popular sites in Washington for camera crews. You'll see why when you explore all the views from this spot. Downstream to the left are the Watergate, Kennedy Center and Roosevelt Bridge. Ahead is Roosevelt Island. Upriver are Key Bridge and the spires of Georgetown. On a sunny day, reflections of the scalloped spans of Key Bridge form a chain of ovals across the river. In freezing weather the Potomac glistens with ice.

Next, you could join the throngs at the Kennedy Center's roof terrace cafeteria for a snack and more spectacular views. But try something different this time. Leave your car in the boat center lot and walk what once was the first half-mile of the C&O's 184.5-mile route up to Cumberland. Take the bike path to the left of the Boat Center as you exit. This becomes the towpath where the restored Canal begins in Georgetown. (Section One of the Towpath Guide to the C&O Canal by Thomas F. Hahn describes this walk from Mile O in depth.) As any hiker will tell you, the view from this level becomes intensely personal and rich in the details of weathered wooden liftlocks and the patterned bricks of Canal-side Georgetown.

A convenient place to stop for refreshments before your return walk is the Fish Market, the only restaurant in Georgetown that has a patio directly beside the Canal. Hikers are welcome indoors as well. A bowl of clam chowder is $2.25; sandwiches are $3.95 to $5.75.

THOMPSN BOAT CENTER -- West side of intersection of Virginia Avenue and Rock Creek and Potomac Parkway. Access via Virginia Avenue westbound or southbound only on the Parkway. Two-hour free parking for Boat Center patrons on upper lot. Fish Market Restaurant of Georgetown, 31st Street on the Canal. Lunch, 11:30 to 4 Monday through Saturday, 11:30 to 2 on Sunday; dinner, 4 to midnight Monday through Thursday, 4 to 12:30 Friday, Saturday and Sunday. ITALIAN RENAISSANCE ON CAPITOL HILL

Most people think "books" when the Library of Congress is mentioned. Relatively few first-time visitors come just to see this remarkable building, perhaps the grandest ever designed by 19th- century American architects. The interior decoration is on such a dazzling scale that a single two-hour visit is only for the once- over-lightly crowd. If savoring at leisure is your style, the Thomas Jefferson Building can be a gourmet feast extended over a number of visits. A guidebook, The Library of Congress: Its Architecture and Decoration by Herbert Small ($9.50 in the Library's giftshops) is indispensable.

For an appetizer:

If Greek mythology is your forte, then you'll be able to identify the huge figures in the fountain at the First Street entrance. (You may be surprised to find some familiar animals splashing among the water deities.)

Bring binoculars so you can better appreciate the details of the 33 unique sculptures that highlight the keystones of the first-floor windows. The guidebook refers to them as "ethnological heads." See if you can locate the Plains Indian and solve the mystery of why he is wearing his feathers horizontally instead of vertically. Next, brace yourself and walk through the impressive bronze doors into the Main Entrance Hall.

If you're bowled over by the magnificence of the richly colored murals and mosaics, the marble and bronze sculptures, you're responding exactly the way the original architects and artisans wanted you to. Gold leaf is applied as lavishly as it was in Venetian palaces. But the proliferation of famous names incorporated into the decoration plan is typically American.

Don't miss the irresistible marble figures of children personified as four continents, halfway up the staircases (and note what's lurking behind "Africa"). The main course must be the Rotunda Reading Room, with its glorious great dome.

By now your neck muscles may need a rest. The descent from Olympian heights to earth can be satisfying if you walk across Independence/Pennsylvania Avenue to the Chesapeake Bagel Bakery and merge into its cheerful hubbub. Coffee and bagels (nine different varieties) for two cost less than $2. Hefty bagelwiches and sinful desserts are offered as well.

THOMAS JEFFERSON BUILDING -- Library of Congress, First Street and Independence Avenue SE. Open weekdays 8:30 a.m. to 9:30 p.m.; weekends, 8:30 to 6. Hours for the rotunda reading room are 8:30 to 5, daily and 1 to 5 Sunday. Capitol South stop on Metro Blue and Orange Lines. Chesapeake Bagel Bakery, 215 Pennsyvania Avenue SE. Open weekdays 7 to 8; Saturday 7 to 7; Sunday 7 to 5.