Occasionally I'll look at the Manhattan skyline and wonder what it was like in 1931 when people around the world were saying, "Holy shit, did you hear what they just built in New York?" Just get up there and trip out about humanity. And be sure to keep an eye out for the unmarked locked door guarding a set of stairs leading to a hidden terrace that was originally designed as a fucking docking station for zeppelins.
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.