The rooftop garden at the Metropolitan Museum of Art. The museum floors below are a maze of tremendous art and inspiration, and taking the elevator to the rooftop garden is the icing on the cake. 360' views of verdant green Central Park bordered by cool glass and steel of Manhattan's cityscape in the distance.
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.