"Shakespeare on Wheels" rolls the work of the Bard downtown Friday and Saturday with free performances of "Twelfth Night" on F Street (which will be closed to traffic) between 13th and 14th NW. Sponsored by The Shops at National Place, WGMS radio and the University of Maryland/Baltimore, the portable Elizabethan theatrics begin at 6:30 Friday, noon and 6:30 Saturday. For the troupe's complete summer schedule call 301/455-2065.


The folks over at the Cystic Fibrosis Foundation's metropolitan D.C. chapter are all wet and want you to be the same at their 1987 Swim Along. To raise money for research, more than 40 YMCAs, clubs and pools will open the lanes for the event from about 9 to noon Saturday and Sunday. For the pool nearest you and for a sponsor sheet (otherwise it's $5 to swim), call 459-8444.


The American Indian Inter-Tribal Cultural Organization Inc. joins more than 20 tribes Saturday and Sunday for its fifth annual Pow Wow at the Garrett County Fairgrounds in McHenry, Md. Beside song, dance, ceremony, arts and crafts, there'll be dance competitions and a couple of social dances with the audience invited to join in. Admission is $4; under 12 free. Call 662-5764 (day) or 963-7284 (evening).


The Mongoose Freestyle Factory Team brings its bicycle- acrobatics to Rockville's Twinbrook Mart Shopping Center for two 45-minute shows at 1 and 3 Sunday. The six-member team performs tricks on the dirtbikes involving ramps, jumps and even the audience. It's free at 2011 Veirs Mill Rd. Call 340-0050.


If last weekend's fireworks didn't fill your bill, boomwise, the Alexandria and USA Birthday Celebration at Jones Point Park is 2 till dark Saturday. There'll be plenty in the way of music, including the Jazz Masters, the U.S. Marine Band and the dixieland sounds of the Band from Tin Pan Alley. After a day full of activities, including children's games, arts and crafts sales and the magic and juggling of the Fearless Airborne Conjury, fireworks will blaze. It's all free at the park, on South Royal Street in Old Town. Call 838-4343 for information.


On July 11, 1864, Confederate troops led by General Jubal Early attacked Washington at Fort Stevens. Here are a couple of events that bring the capital combat back to life:

Rock Creek Park commemorates the battle beginning at 10 Saturday with a 30-minute slide show at Rock Creek Park Nature Center, 5200 Glover Rd. NW. The rest of the day includes tributes, a wreath-laying ceremony, music by the Patuxent Brass Band, tours and a look at 19th-century medical practices. Everything, except the slide show, is at Fort Stevens, 13th and Quackenbos NW, and it's free. Call 426-6829.

The Fort Ward Museum and Historic Site in Alexandria remembers the battle with a program of camp life and military drills from 1 to 4 Saturday; torchlight tours of Confederate and Union camps at 8 Saturday ($2); preparations for the battle at 11 Sunday; the battle of Fort Stevens reenactment at 2 Sunday ($2). The 45-acre park is at 4301 West Braddock Rd. Call 838-4848.


Pick up the beat without breaking the piggy bank at these free concerts: The Netherlands Carillon (near the Iwo Jima Memorial) rings out at 6:30 Saturday. 285-2598 . . . The Rudy Adler Jazz Band plays dixieland 7:30 to 9 Friday on the Montpelier Mansion grounds, Rte. 197 and Montpelier Drive in Laurel. 953-7882 . . . Doc Scantlin and his Imperial Palms Orchestra swing 1 to 3 Saturday at the Plaza del Mercado off Bel Pre Road in Silver Spring with free jitterbug lessons and a tango contest. 598-8440 . . . The Concerts in the Bandstand series is at 4 every Sunday until September 27 on the West Lawn of the Smithsonian's Museum of American History. This weekend it's Brazilian carnaval music and the dancers of BrasUSA. Call 357-2700 . . . the Post Nuclear Jazz Society plays at 7 Saturday at Fort Hunt Park, George Washington Parkway and Fort Hunt Road, in Alexandria. 285-2598.


It's a Country Folk Art Show and Sale 5 to 9 Friday, 10 to 8 Saturday and 10 to 5 Sunday at the state fairgrounds in Timonium, off I-83. Admission is $5 Friday, $3 Saturday and Sunday. Call 313/634-4151 . . . The National Museum of American History's Mail in Motion program is 1 to 4 Saturday with concerts, demonstrations, slide shows, the issuing of a special postmark (see stamp column on page 45) and more. Free. Call 357-2700 . . . Berrywine Plantation's Razz Jazz Cajun Festival is noon to 5 Saturday and Sunday with Louisiana delicacies, Ed Lehmann and his jazz band and tours of the winery ($2); 13601 Glisans Mill Road, north of New Market. Parking is $2. Call 301/662-8687 for information.


Forget all your worries at the seventh annual Lotus Blossom Festival 11 to 5 Saturday and Sunday at the Lilypons Water Gardens in Frederick, where homage will be paid to the legendary flower. Besides an abundance of gardening advice, entertainment includes cabaret and theatrical groups and local bands. Don't get too full at the pie-eating contest (at 3 both days) because plenty of Mediterranean and Oriental food will be for sale. Admission's free; parking is $2. Call 428-0686 for information.