Cool down this Saturday at Rockville's "Christmas in July Fair." More than a hundred artisans will be on hand to display their wares; the Village Jazz Band performs most of the afternoon; and there'll be plenty of chow and refreshments. Santa Claus is also slated to work up a sweat taking orders from kids to take back to the North Pole.

Admission to the following activities is free unless otherwise noted.

SENIOR CITIZEN PARTY -- Friday, 10 a.m. to 2, Good Luck Community Center, 8601 Good Luck Rd., Lanham. Dancing, ceramics, exercise and needlework. Bring a lunch and a friend. 552-1093.

STEAM CITY -- Friday-Sunday, 9 a.m. to midnight, Clarke County Fairgrounds, Berryville, Va. (U.S. 50 west to Rte. 340 south into Berryville; make left on Business 7 west; fairgrounds one mile on right), "Pageant of Steam." A weekend of old steam engine exhibits, live bluegrass music, flea markets and more. Adults $2; children under 15, free. For more information call 703/635-3928 or 837-1140.

OLD TOWN TOUR -- Friday, 8 p.m.; Saturday, 10:30 a.m., 8 p.m.; Sunday, 2, Ramsay House Visitors Center, 221 King St., Alexandria. Take a special summer tour retracing the steps of George Washington and Robert E. Lee with a colonial-attired guide through the cobblestone streets of Old Town Alexandria. Adults $2; ages seven to 15, $1. 548-0100.

NATIVE AMERICAN PRESENTATION -- Friday, 8:30 p.m., Campfire Circle, Greenbelt Park, 6565 Greenbelt Rd., Greenbelt. (Baltimore-Washington Parkway to Greenbelt Exit; bear right on Greenbelt Road through first light, park entrance is 500 feet on left; follow signs to campground). A native American Indian in tribal costume will relate the legends of the Penobscots. 344-3943.

CRAFT SALE -- Saturday, 10 to 4, Courthouse Square park, Rockville (in Town Center), "Christmas in July Sale." More than a hundred artists and craftsmen display a wide variety of handicrafts; live jazz from 12:30 to 3:30; food. 424-8000, ext. 337.

STREAM STROLL -- Saturday, 10 to 11:30 a.m., Visitors Center, Ellanor C. Lawrence Park, Centreville, Va. (take I-66 west 11 miles to a left on Sully Road, Rte. 28; turn immediately right on Walney Road; park entrance one mile on left). "Life Along a Stream." Join a naturalist for a close-up look at the importance of streams and their relationship to the life cycles of plants, people and animals. Wear sturdy shoes and clothes appropriate for hiking. 631-0013.

BIRCHWOOD CITY CELEBRATION -- Saturday, 10 to 7:30 p.m., Birchwood City Recreation Center, 1331 Fenwood Ave., Oxon Hill. (Rte. 210 south, head east on Oxon Hill Road then make left on Livingston Road going four blocks to right on Birchwood Drive; turn left on Dunwoody; follow signs tor recreation center). "30th Annual Celebration of Birchwood Cit." Flea market, games, activities, entertainment, break dancing, food and more. 839-2132.

MMMMMMMARYLAND -- Saturday, 11 to 3, Maryland Outlet Mall, 7351 Assateague Dr., Jessup (take I-95 north to Columbia exit toward Fort Meade; mall is 1/2 mile at intersection of Rte. 175 and U.S. 1), "Favor the Maryland Flavor." A day featuring produce stands, continuous live music, raffles and watermelon and peach-eating contests. 301/799-4077.

LIVING HISTORY -- Saturday and Sunday, 1 to 4, Fort Frederick State Park (20 miles west of Hagerstown off I-70 to Rte. 56 at Big Pool). Maryland forces bring the fort alive with a reenactment of marching, drilling and training in frontier Indian tactics during the French and Indian War. 301/269-3761.

RECORD COLLECTORS' SWAPMEET -- Saturday, 6 to 10 p.m., St. Matthews Church, 36th Avenue and Nicholson, Hyattsville (two blocks south of Queen's Chapel and East-West Highway). Hundreds of LPs in many styles will be available for trading and selling. $1 admission. 521-2327.

LAWN PARTY -- Sunday, 2 to 4:30, Smallwood's Retreat, Smallwood State Park, Route 224, Marbury, Md. (take Rte. 210 south 19 miles to Rte. 225 east for one mile; make right on Rte. 224. Follow signs, park two miles on right). Visit the restored home of Major General William Smallwood; see a colonial soldier demonstrate his Brown Bess musket and join in a festive lawn party with lemonade punch and an assortment of colonial cookies; you can also watch a park ranger make a dried flower arrangement. 301/743- 7613.