Riverbank State Park. I have an almost identical photo to this one above somewhere in my photo archives (I haven't taken the time out to look for it). I often pass by Riverbank State Park on my longer bike rides through Manhattan. The park is situated along the Hudson River between 137th Street and 145th Street. The park itself is unusual because it lives on top of a sewage treatment facility (don't worry though, there's not really any sort of smell except those scents coming off of the Hudson River).
Anyway, I really like this park because of its high vantage point of NYC and New Jersey across the river. And because of its unusual location, the park itself is often quite empty even in the heart of summers when its pool, roller rink, field, and other outdoor facilities are open to the public. For me, it is a place to go escape from the city while still being in the city itself.