The Soap Box Derby mentioned in "Weekend's Best" of today's Weekend section does not take place tomorrow, but on Saturday, July 25. (Published 7/17/87)

MODELS AND ROCKETS The seventh annual Model Rocket Contest at the NASA/Goddard Space Flight Visitors Center lifts off at 10 Sunday. More than a thousand rocketeers will compete in events measuring flight time and accuracy. Spectating and on-site registration are free at the center on Soil Conservation Road in Greenbelt. 286-8981.

From Porsches to "Top Gun" fighters to battleships, the Washington Chapter of the International Plastics Modelers Society displays hundreds of miniature replicas 12:30 to 5 Friday and Saturday at the Twin Bridges Marriott in Arlington. Admission is $2, $1 under 12. Call 369-6784.

A SPIKE IN TIME Have a smashing good time at the fifth annual National Kidney Foundation's Volleyball Marathon, 8 to 8 Saturday on the Mall between Fourth and Independence SW. The Montgomery Volleyball Association will give free serving and spiking clinics and DC 101's disc jockeys will take on the Redskinettes alumnae before they play whichever team raises the most money. Spectating is free. Team registration is 9 to 5 Friday; sponsorship is on a pledge-per-point basis. 337-6600.

ALL KEYED UP The University of Maryland's International William Kapell Piano Festival and Competition comes to a melodic close this weekend with two concerts: Austrian Paul Badura-Skoda in works by Mozart, Schubert, Beethoven and Berg 8:30 Friday at the campus' Tawes Theater. Tickets are $12 ($10 for students and seniors), at the door. Call 454-2201 or 454-5276.

On Saturday the three finalists of the competition perform with the National Symphony Orchestra conducted by Sixten Ehrling at 8:30. Tickets, $12 to $25, are available at the Kennedy Center Instant Charge, 857-0900. (WGMS 570AM/103.5FM will broadcast both concerts live.)

There's more competitive music at the Kentucky Fried Chicken Gospel Music finals Sunday. Eight finalists -- two each in choir, ensemble, male and female -- will compete at the Kennedy Center Concert Hall at 7. Pop and gospel singer Deniece Williams performs afterwards. Tickets are $9.50 to $10.50. Call 842-3455.

THE LATIN BEAT The Smithsonian's "Tarde Hispana" (Spanish Afternoon) brings Latin music and culture to the museums 4 to 9 Sunday. There's dance music by the Maria y sus Magnificos Orquesta in the American History's bandstand on the West Lawn; films at 5:45 in the Carmichael Auditorium; Tex-Mex band Little Joe y La Familia at 7:30 on the Carmichael stage and bilingual museum tours at 4:30 to 9 (Air and Space) and 5 and 6 (Natural History). Free buses from Lincoln Junior High in Mount Pleasant to the museums run at 3:30 and 4:30 and return at 8:15 and 9:15; call 576-7582 on Friday and 673-6826 on Saturday to reserve a seat. 357-2700.

REST OF THE BEST The Tri-State Photo Fest is 10 to 4 Sunday at the Silver Spring Armory, 925 Wayne Ave. $3.50. 201/533-1991 . . . the Washington Soap Box Derby finals are 9 to 4 Saturday at Eastern Avenue and Varnum NE. Call 279-7174 . . . Free two-mile City of the Constitution Walking Tours from Western Plaza to the Lincoln Memorial are 10 to noon Sunday -- and every Sunday through August. For reservations, call 426-6770.

NOTES TO NOTE Summertime and the listening is easy, especially at these free concerts: The Bob Lewis Big Band puts you in the mood at Fort Hunt Park, on the GW Parkway in Alexandria, at 7 Saturday. Call 285-2601 . . . Ironweed performs English ballads at 1 Saturday at the Old Stone House, 3051 M St. NW in Georgetown. Call 426-6851 . . . The U.S. Air Force Concert Band continues its summer series at 8 Friday at the Sylvan Theater. Call 767-5658 . . . Betty Carter and the Lavern Reed Dancers join talents at Fort Dupont, 8:30 Friday and Saturday. Call 426-7723.

ARTFUL EVENTS The 21st annual Art in the Park festival showcases the work of visual artists noon to 7 Sunday at Malcolm X/Meridian Hill Park at 16th and Florida NW. There's photography, painting, sculpture and handicrafts and the Trinidad and Tobago Steel Band, the African Heritage Dancers and Drummers and the D.C. Youth Ensemble, among others, will perform. Call 829-1188.

Baltimore sizzles with free entertainment at Artscape '87: exhibits, children's activities and theater, and non-stop music. Highlights include George Howard on jazz saxophone Friday at 8:30, blues singer Leon Redbone Saturday at 3:30, Art Blakey and the Jazz Messengers Sunday at 8:30. The festival is 6 to 11 Friday, 11 to 11 Saturday and Sunday on Mount Royal Avenue between Preston and Lafayette streets. 301/396-4575.