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 New York Earth Room is an interior sculpture by the artist Walter de Maria that has been installed in a loft at 141 Wooster Street since 1977. Go see!