IN TOWN The United States Air Force Band continues its free Guest Artist Concert Series Sunday at 3 with Richard Carpenter hosting. The concerts are at DAR Constitution Hall, 18th and D streets NW. 767-5658 . . . The Bethune Museum and Archives hosts a performance by pianist Percy Gregory this Sunday at 3. Gregory will play works from such composers as Haydn, Liszt and Chopin. The museum is at 1318 Vermont Avenue NW. Admission is $1; 332-1233. IN MARYLAND

Tap a tree at Brookside Nature Center's Ninth Annual Maple Sugar Festival this Saturday. There'll be free films, lectures and a maple syrup-making demonstration at 11, 1 and 3. The center is off Glenallen Avenue in Wheaton Regional Park. 946-9071 . . . Operafest '86 kicks off with a Catholic University production of Puccini's "Gianni Schicchi" and "Suor Angelica" Saturday at 8, Sunday at 7:30. The festival runs through March 8 at Montgomery College's Performing Arts Center, 51 Mannakee Street, Rockville. Tickets are $7; $5 for seniors and students. 279-5301. IN VIRGINIA

You can see thousands of comic books, including an original 1939 Superman, at the Comic Book and Science Fiction Convention this Saturday from 10 to 5 at the Holiday Inn, 1489 Jefferson Davis Highway, Arlington. Admission is $3; 301/292-4587 . . . Take a musical world tour with the Arlington Metropolitan Chorus at its International Songfest Sunday at 3 at the Westover Baptist Church, 1125 N. Patrick Henry Drive at Washington Boulevard, Arlington. Tickets are $6; $4.50 for students and seniors. 558-2165 . . . The Jazzmasters perform a free concert of Basie, Goodman and Miller Sunday from 2 to 4 at Lee Recreation Center, Lee District Park, 6601 Telegraph Road, Rose Hill. 922- 9841.