One of the only things I enjoy about taking the subway these days is seeing the defaced ads on the L train. Whether it's random chance, high schoolers or street artists the result is always good.
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.
The Frank Stella's are the best. They are all over the city and they always look amazing. Two of my favorites are the Saatchi and Saatchi lobby (pictured) and the Citi Corp building in Midtown.
The best part of the new Whitney Museum is the view in all directions from the terrace - and also from the staircase facing West. I'm afraid it upstages the Art.