It's a sight to behold. The mountains (glorified hills really) have colors that you wouldn't think possible - nothing like the earthtone tinged reds, yellows and oranges in Europe. I remember my parents were driving down to the Cape their first fall in the US when they heard on the car radio about some Japanese tourists coming over to see the fall colours in New Hampshire. They promptly turned around and went north - they didn't regret it.  There's a reason why the Times has a leaf map every day during the fall.

NYC itself is nowhere near as nice but you can get some good colours just a bit north of here along the Hudson valley (gorgeous river valley in all respects). Riding up north along the river to Bear Mt. (40-50 mi. from NYC) is a wonderful early October bike ride. If you have the endurance continuing up to West Point is great as well. The freight train that runs along the same route isn't so picturesque.