SEVERAL SHOWS in town are enjoying a second, or even third wind: The Shakespeare Theatre at the Folger is extending "The Merry Wives of Windsor" again, through July 8. This will be the last extension, as cast members, including Pat Carroll, who plays Sir John Falstaff, have other committments. Carroll is playing Big Sister in "Nunsense" again this year, at East Carolina University, in Greenville, N.C. For "Merry Wives" information, call 546-4000.

Washington audiences also have embraced the Studio Theatre's production of Terrence McNally's "Frankie and Johnny in the Clair de Lune," a touching, comic romance between two extraordinarily ordinary people. So Studio says Frankie and Johnny can stay till July 8. Call 332-3300.

And Smallbeer Theatre company is stretching Dale Stein's stay through June 23; Stein's one-woman, multi-character social satire "A Fresh of Breath Air" is at the Church Street Theatre (formerly American Playwrights Theatre) at 1742 Church St. NW. Sunday's performance benefits the Smallbeer troupe, one of the city's most inventive companies, and is followed by a reception at the theater. Call 265-3748.

SEVERAL Washington comedy acts and musical artists are chipping in their time and talent to help homeless kids: On Monday at 7, the topical troupes Mrs. Foggybottom and Friends and Fresh Victims, plus baritone Charles Williams and Schubert pianist Frank Ruppert, will present "A Celebration for Children" at the Marquee Cabaret at the Omni Shoreham Hotel. Call 949-1102.

THE SATIRICAL comedy troupe Gross National Product is now producing "electronic political cartoons" for syndication on news and commentary shows around the country; the troupe's efforts can be seen on WETA Channel 26, Saturdays at 1 a.m., on "The '90s," a series showcasing independent video work from around the world. GNP continues to knock the newsmakers live at the Bayou on Saturdays at 7:30. Call 783-7212.

ARENA STAGE is selling off its some of its props, and among the dozens of items up for sale are the throne used in "Enrico IV," the Russian garden benches used in "Nothing Sacred" and the patio furniture from "Cocoanuts." Sale hours are 10 to 4 Saturday in the stage lobby at Sixth Street and Maine Avenue SW. Admission is free; call 554-9066.