The permanent piece called "Meeting" at MoMA PS1 is an etherial experience of light and color and quiet. Sit and look through the cutout in the ceiling and watch the light change both outside and in. You can't tell what you're looking at until maybe a bird flies by.
The best underground shows you will find will have free pbr and models as far as the eye can see. They are rarely announced, so keep your eyes out for promising openings, especially with Milk Underground coming up.
Has no sign outside, so you might miss it, but if you do happen to find it, you will feel like you have walked into the 1920's. Its very chic, with Black and white marble decor, candle light and ambience to go with a great list of cocktails and whiskeys etc.