Sunday brings an unusual variation on the garage-sale theme. The Source Theater Company, which lost its Resource Theater and, with it, much of the storage space for old props and costumes, is doing a bit of spring cleaning. The theater's backstage debris may just be your treasure. Prop up your living space with such fading stars as a lamp, a chair, some books, a few records, or dress up with a mink coat.

Admission to the following activities is free unless otherwise noted.

USED BOOK SALE -- Friday and Saturday, 10 to 4; Sunday, 1 to 4, Oatlands Plantation, Carriage House, U.S. 15, Leesburg. The annual used book sale benefits the restoration of Oatlands' antique collection. 703/777-3174.

ST. SOPHIA SPRING FESTIVAL -- Friday through Sunday, noon to 9, St. Sophia Greek Orthodox Cathedral, 36th Street and Massachusetts Avenue NW. An explosion of things Greek: food, pastries, beverages, music, games, raffles, tours of St. Sophia's Byzantine Cathedral, "Free Phone Calls to Greece," and more. 333-4730.

17TH-CENTURY ST. MARY'S CITY -- Saturday and Sunday, 10 to 5, Historic St. Mary's City, Maryland Rt. 5. Colonial games at the Godiah Spray Tobacco Plantation and wild foods at Farthing's Ordinary are scheduled for this weekend. 301/862-9880.

PATUXENT RIVER DISCOVERY -- Saturday, all day, along the hundred-mile length of the Patuxent River. Discover, clean up and appreciate river resources with raft races, canoe trips and hikes. 627-6074.

BIRD AND BREAKFAST -- Saturday, 7:30 to 9:30 a.m., Hidden Pond Nature Center, 8510 Greeley Blvd., Springfield. Have continental breakfast and search for spring migratory birds with Park Authority naturalists. Beginners and experts invited. $2. 451-9588.

MIGRATING WARBLERS -- Saturday, 9 a.m., Rock Creek Nature Center, 5200 Glover Rd. NW. Join a park ranger for a one-hour peek at these little birds on their way to their northern breeding grounds. 426-6829.

AUTO EXPO -- Saturday, 9 to 5, Prince George's Equestrian Center, intersection of Routes 4 and 301, Upper Marlboro. More than 200 antique cars, hot rods and pre-1970 convertibles. Adults $2; children under 12 free. 887-8288.

ALEXANDRIA AZALEA FESTIVAL -- Saturday, 10 to 5, Fort Ward Park, 4301 West Braddock Rd., Alexandria. Flowers plus arts and crafts exhibits, bluegrass and country music, international food. 838-4343.

CHINESE SPRING FESTIVAL -- Saturday, 10:30 to 5, Chinese Community Church, 1011 L St. NW. Dim sum lunch and Chinese folk dancing. 289-3611.

GYPSY MOTH -- Saturday, 11 a.m., Rock Creek Nature Center, 5200 Glover Rd. NW. Short film and discussion about this serious insect pest. 426-6829.

PLANTS GROWN IN HANGING BASKETS -- Saturday, noon and 2 p.m., U.S. Botanic Garden Conservatory, First and Maryland Avenue SW. One-hour class on growing plants in hanging baskets. 225-7099.

BASEBALL BASH -- Saturday, 4 p.m., Danker's Sports Bar, Sixth and D streets NW. Baseball cards and memorabilia on sale from 4 to 8; meet former Washington Senator's pitcher Dick Bosman and some ex-teammates. Sponsored by the Washington Senators Fan Club and DC 101. 828-9932.

SQUARE DANCE -- Saturday, 8 p.m. to 10:30 p.m., Arlington Forest Methodist Church, U.S. 50 and N. Henderson Street, Arlington. Live music with caller Tom Hinds. Beginners and singles welcome. $4. 430-9006 or 241-0993.

VINYL EVENT -- Sunday, 10 to 6, Holiday Inn Crowne Plaza, 1750 Rockville Pike, Rockville. Jazz, punk, bluegrass, new wave and more records. $2. 202/462-6178 or 301/598-2040.

DRAMATIC GARAGE SALE -- Sunday, 10 to 3, Warehouse Rep Theater, 1835 14th St. NW. The Source Theater offers its props and cast-offs for sale. 462-1073.

SPRING WILDFLOWERS -- Sunday, 10 a.m., Rock Creek Pa, Boundary Bridge, Beach Dr. (DC/MD line) NW. Search for spring wildflowers on this two-hour hike with a park naturalist. 426-6289.

PICNIC FOR PEACE -- Sunday, noon to 3, Kalorama Park, Kalorama and Columbia roads NW. Women's Action for Nuclear Disarmament sponsors a Mother's Day Picnic for Peace. Bring your own picnic. 387-6885.

WATERFALL HIKE -- Sunday, 3 p.m., Nature Center, Prince William Forest Park, Rt. 619 West (a half mile off I-95), Prince William County. Join a park ranger for a moderately paced hike along the North Branch of Quantico Creek. 703/221-2104.