Calling all bibliophiles: The 13th annual Washington Antiquarian Book Fair is 10 to 6 Saturday and noon to 5 Sunday at the Rosslyn Westpark Hotel where 70 exhibitors will have thousands of old books, manuscripts, autographs and other printed collect- ibles for sale. The hotel is at 1900 North Fort Myer Drive in Arlington. Admission is $3. Take Metro's Blue/Orange line to Rosslyn. Call 654-2626 or 527-4814.
MARYLAND IS FOR KIDS
Sunday's a great day to take the kids to the eighth annual Children's Fair at the Maryland State Fairgrounds exhibition hall in Timonium. Kids 4 to 14 can see plays and magic shows by Jafa (the Magical Minstrel) and performances by Israeli, Irish, Balkan and Polish folk dancing troupes and the Hanover Ballet between 11 and 5. There'll also be clowns, face-painting, storytelling and more than 40 participatory arts and crafts booths. Admission is $2.50 and almost all of the activities are free. Call 301/225-0052.
MEASURE FOR MEASURE
The Levine School of Music opens its doors for the musical marathon "Playathon" 1 to 5 Sunday. Tuneful events include a Voice-In -- a singalong with Metropolitan Opera baritone Charles Williams -- and a Music Theory Carnival (educational games of note, or at least about notes). Records and food will also be for sale. Plus Levine's "Classical Plus" series starts Friday with the Lenox Ensemble's improvised folk, rock, jazz and classical fare at 8. (The series continues through March.) The school's at 1690 36th St. NW, and both the Playathon and Friday's concert are free. 337-2227.
REST OF THE BEST
Deborah Benner and the Tropic of Capricorn, her Brazilian-Jazz fusion group, perform at the Brazilian-American Cultural Institute (4103 Connecticut Ave. NW) for two shows at 8 and 9:30 Friday. Admission is $6 and $8. Call 362-8334 for information . . . the 25th annual Woodlawn Plantation Needlework Exhibition opens 9:30 to 4:30 Saturday and continues daily until March 27. Admission is $5; $3 for kids 6 to 17 at the intersection of U.S. 1 and Rte. 235 in Mount Vernon. Call 780-4000 . . . the free U.S. Air Force Guest Artist Series continues with the Airmen of Note and jazz vocalist Diane Schuur at 3 Sunday at D.A.R. Constitution Hall, 18th and D streets NW. Call 767-5658 . . . Vocalist Arnae', pianist Aaron Graves, bassist James King and drummer Nasar Abadey play a concert of composer Ida Cox's music at 1:30 and 3:30 Sunday in the Palm Court of the Museum of American History. Free. 357-2700.
A DEUX This is the weekend for ethnic shows at Georgetown University's Gaston Hall:
At 7 Friday, the Bayanihan Philippine Dance Company combines Filipino folk art with stylized theater. Tickets are $10; proceeds go toward community-based self-help projects in the Philippines. Call 569-2269.
The mood moves to the Near East at 7:30 Saturday with "Traditional Persian Music: the Art of Improvisation," with Mohammed Reza Lotfi and his ensemble performing on tar, setar, daff, tonbak and voice. Tickets are $15 at the door. Call 493-5881.
The 278-foot research vessel Malcolm Baldrige opens its decks for guided tours 10 to 4 Saturday and Sunday at the Washington Navy Yard, Pier One, near the Navy Museum and the Marine Corps Museum. The 2,963-ton boat is one of the National Oceanic and Atmospheric Administration's largest. Free. Call 377-8090 or 433-2120.
AMONG THE STACKS
To be heard at the Library of Congress this weekend:
Violinist Charles Treger plays a "Hartmann" Stradivarius, made in 1723, accompanied by pianist Menahem Pressler in works by Faure', Mozart and Feldman at 8 Friday. Call 287-5502.
Same room at 5:30 Saturday, the Meliora Quartet, winner of the 1985 Naumburg Chamber Music Award, performs works by Beethoven, Mendelssohn, Barto'k and others.
BIG LITTLE SHOW
The fifth annual Dollhouse and Miniatures Show and Sale brings all things small and smaller to the Crowne Plaza Hotel (1750 Rockville Pike) 10 to 5 Sunday, including dolls, furnishings, paintings, porcelain, needlework and the log cabins and Victorian mini-mansions to put them in. Admission is $3; $1 for kids under 12. Call 978-5353.
More than 180 dealers will show off their golden oldies -- everything from sterling service to art work to furniture -- at the D.C. Antiques Show noon to 9 Friday and Saturday and noon to 6 Sunday at the D.C. Armory. Admission is $4 (for all three days), kids under 14 are free with a grown-up. 547-9215.
THE GRASS GROWS
Fairfax High School is the place for WAMU-FM's Bluegrass Winter Festival concert at 8 Saturday. The concert includes the Seldom Scene, Hot Rize, Red Knuckles and the Trailblazers and Del McCoury and the Dixie Pals. Tickets $15 at 3500 Old Lee Highway. Call the station for information. Call 885-1030.
The Winter Olympics are done but competition is alive and well this weekend at the 1988 McDonald's American Cup Gymnastics Championships at George Mason University's Patriot Center. Some of the world's top gymnasts will compete, including 1987 U.S. National Champions Scott Johnson and Phoebe Mills, three-time Olympic gold-medalist Li Ning from China and World Champion Sylvio Kroll from East Germany. Opening day competition begins at noon Saturday ($8.50 and $10.50); the closing round's at 2:30 Sunday ($11 and $13). Call 323-2672. Charge tickets at 432-0200.