It took us about three and a half hours to tour the whole grounds and check everything out. If you're looking for a fun day trip from NYC, I highly recommend it. After we'd finished exploring, we just sat on the hill for a while people watching and waiting for the sun to go down.
Even further across the park, are some interactive sculptures! This piece had a bunch of heavy rubber-mallets laying all over the place and could use them to wack the sculpture in different places and it would resonate in all sorts of weird ways. It was really neat.
On the other side of the art center are a bunch of enormous crane-like sculptures that are made out of steel i-beams. This one must have been like 4 storeys tall!