This is the worst spot in the city and the most popular at the same time, you should avoid it at all costs. All that you could find here is a giant open space on smooth basketball court ground where people usually bring boxes and rails to from Autumn Skateshop, half a block down on ninth street. Stay away.
Unfortunately, you never get kicked out of here. The boxes are typically only set up between 1 P.M. and 6 P.M.
9th Street and Avenue A. Take the L to First Avenue, skate south on First Avenue until you reach 9th Street, make a left, and skate east for a block until you reach the park.