Myself and Mrs Bear went there last year for five days, could have done with longer as there is so much to take in.The Empire State building is good at night for the lights, there is an aircraft museum on board the aircraft carrier Intrepid beside pier 84 which has a space shuttle on its deck as well as the SR71 Blackbird and Concorde parked on the dock beside it plus loads to see inside. The Rockefeller center is good for a high speed lift ride to the top and great views.
The American Museum of Natural History is definately worth a view but you will need the best part of a day to see it all, as Friar said visit Grand Cental its mind blowing and the 911 museum is a must, a water taxi ride is good, not quite as mad as the yellow cab ride LOL There is so much to see and do there so its on the bucket list to go again but for longer, have a great time when you go i know we did.