We're going to NYC for a few weeks in December with our three year old and we're looking for some (preferably free) indoor places she can run around and explore. In London we have the Royal Festival Hall, which has lots of seating and open space, a cafe, bar, lots of stairs and lifts and is open to the public and very child friendly. You can bring your own food, there's often free live music, etc. Basically, I'm looking for a place just like the RFH, but in Brooklyn or Manhattan.
Any suggestions (apart from museums, as I have that covered) are most welcome.
I can't wait!