One of the activities I enjoyed the most was going to Summer Movie Nights in Bryant Park. Surprisingly, even if they are free, the movies played are really good ones no matter they are not brand new. As a drawback, since it is free you would have to get there early enough to find a spot because the lawn gets crowded so fast. Some people even picnic there before the movie to kill the time while being much earlier in advance to the movie show.
For me, one of the most amazing features was the context. You are enjoying a movie right in the heart of Manhattan, surrounding by all those skyscrapers that appear behind of the trees.
Bryant Park was a very convinient location for me since my office is right there, but in summer time there are open air cinemas in most of New York Parks, such as Central Park, Prospect Park, Mcrren Park, and so on