Tompkins Square Park -- wait and see a/b the rain!

7th Street and Avenue A.

F train to 2nd Avenue
Walk north to 7th Street, then turn right.
Walk to the center of the park parallel to 7th Street
OR
N, R, to 8th Street
6 to Astor Place
Walk east along 8th Street until you hit the park.
Walk to the center of the park parallel to 7th Street.