It's time for some good clean fun this Saturday at the 11th Annual Mid-Atlantic Championship Bathtub Race at Maryland's Point Lookout Park. Approximately 20 motorized bathtubs will vie for speed titles in several classes (including one strictly for 200-pound skippers).

Watching should be fun enough; but if you get the hankering to rip out your tub, you can still enter the day of the race. (It might be wise to contact park officials first about rules and regulations).

The following events are free unless otherwise noted.

RENAISSANCE FESTIVAL -- Saturday and Sunday, 10:30 to 6, Crownsville, Md. (take U.S. 50 east, then go north on Rte. 3, east on Rte. 450 to Crownsville Road, turn left for one mile to the festival site). Jugglers, jousters and magicians along with other 16th-century characters come alive in this salute to the Renaissance. Adults $7.50; Seniors $6.50; 13 and under $3.50; five and under free. 261-1970

STEAM AND GAS ROUNDUP -- Friday-Sunday, 10 to 5, Carroll County Farm Museum, 500 South Center St., Westminister, Md. (Take Georgia Avenue which becomes Rte. 97; in Westminster turn left on Rte. 32; go 2.2 miles to Center Street, make left; museum 1/2 mile on the left). See antique farm machinery, steam and gasoline engines; flea market, food and entertainment. 301/848-7775.

ANTIQUE SHOW AND SALE -- Friday, 11 to 9:30; Saturday, 11 to 8, St. Alban's School for Boys, National Cathedral Close, intersection of Massachussetts and Wisconsin avenues NW. More than 50 dealers display their wares from the 18th and 19th centuries. $4 admission. 537-9065 or 703/780-9200.

GIANT FLEA MARKET -- Saturday, 9:30 to 2, in the alleys between S & T and 35th & 37th streets NW, "8th Annual Burleith Flea Market." Hundreds of items, including furniture, housewares, toys and children's clothes. 338- 6680.

OUTDOOR MARKET -- Saturday 10 to 4, Field House parking lot, George Mason University, 4400 University Dr., Fairfax (at the corner of Rte. 123). More than a hundred vendors with antiques, crafts, jewelry, household collectibles and more. 323-2966.

BOOK SALE -- Saturday, 10 to 2, Bowie Branch Library, 15210 Annapolis Rd., Bowie. Look over a wide variety of paperbacks, magazines and recordings; most items under $1. For more information and directions call 699-3500.

CARNIVAL AND FLEA MARKET -- Saturday, 10 to dusk, Hemingway Memorial Church, 5252 Addison Rd., Chapel Oaks, Md. (take Rhode Island Avenue to South Dakota Avenue, make a right; follow signs to Eastern Avenue and Addision Road merging right; make left at light, then make another left on Addison Road at next light; church is on left at third light), "2nd Annual Carnival/Flea Market." Flea market plus barbecue and baked goods. 925-9634.

KOI FESTIVAL -- Saturday-Sunday, 11 to 5, Lilypons, Md. (Take I-270 north 25 miles to exit 7 at Urbana, make right on I-80 west for 11/2 miles. Make left on Park Mills Road for three miles to left on Lilypons Road for 1 mile). A two- day celebration of the Japanese carp; jousting, entertainment, canoe rides, arts, crafts and food available. $2 donation parking fee is optional.301/874-5133.

RUB-A-DUB-DUB -- Saturday, noon to 3, Lake Conoy, Point Lookout State Park, Scotland County, (From the Beltway take Rte. 5 Rte. south past Waldorf and St. Mary's to the very end), "11th Annual Mid-Atlantic Bathtub Race." Take part in or just watch the fun and excitement of bathtub racing; pre-registration encouraged but not necessary. Entries taken day of race. $10; looking's free. 301/872-5688.

SERBIAN PICNIC -- Saturday, 2 to 8, St. Luke Serbian Orthodox Church, 6801 Georgetown Pike, McLean (Take George Washington Parkway to McLean exit; get on Rte. 123 , exit on Rte. 193 west. Pass CIA building, go two miles, make left on Douglass Drive; church on the corner). Traditional Serbian food, music and dance. Adults $2; under 16 free. 821-1394

INTERNATIONAL GARDEN PARTY -- Saturday, 5 to 8:30, Sixth Presbyterian Church, 16th and Kennedy streets NW. All kinds of international food, reggae music, Korean dancers and more. Adults, $7; children, $3.50 (includes all the food you can eat and entertainment). 723- 5377 or 829-7121.

FESTIVAL ITALIANO -- Sunday, 11 to 6, Villa Rosa Grounds, 3800 Lottsford Vista Rd., Mitchellville, Md. (take U.S. 50 east toward Annapolis to exit 704 N which leads to Lottsford Vista Road), "24th Annual Italian Fall Festival." Includes flea market, games, prizes, music and traditional Italian food. 301/459-4700.

FOOLS GOLD MINING -- Sunday, noon, Park Headquarters, Prince William Forest Park, near Quantico, Va. (from I-95 take exit 50-B, Rte. 619 west which leads to park). Join a ranger for a three-mile hike to find an old pyrite mine. 703/221-2104.