An hour away from NYC, there lies a summer camp on a pristine lake which was abandoned 15 years ago. I cant tell you where this is, but some research can lead you to the right place.
My second-favorite outdoor-place in Manhattan. The abandoned 1.6 km rail tracks running above Chelsea down to the Meatpacking district were turned into an above-ground park. Perfectly nice place for sitting in the sun, reading, relaxing, having lunch in some green spot in the city. Nice views at the Hudson River, New Jersey, the Statue of Liberty (little small from there but still visible) and the architecture around Chelsea.
Wonderful place to grab a coffee+pastry with a great vibe and outdoor-only seating. Best cold-brew in the neighborhood. Doubles as the headquarters for The Lot Radio station, so there is often a live DJ spinning pretty chill music.