A pioneer in the "alternative-space" movement, P.S.1 rose from the ruins of an abandoned school in 1976 as a sort of community arts center for the future. Now a partner of the Museum of Modern Art, P.S.1 still focuses on community involvement, with studio spaces for resident artists and educational programs. In summer, its Sunday afternoon outdoor dance parties with fashionable DJs attract a crowd in which everyone looks like an art student. P.S.1's exhibition space is enormous, and every available corner is used -- four floors, rooftop spaces, staircases and landings, bathrooms, the boiler room and basement, and outdoor galleries. Exhibitions reflect the center's mission to present experimental and formally innovative contemporary art, from the progressive and interactive to the merely incomprehensible. It's never dull. COST: $5 suggested donation. Subway: 7 to 45 Rd.-Courthouse Sq.; E, V (weekdays only) to 23rd St.-Ely Ave.; G to 21 St.-Van Alst.