It was definately the Empire State Building. Built in the 1930s, it's still one of the world's tallest buildings. The Circle Line Cruise was also a very great thing to do. They offer a few different length tours.
Quick Tips:
Some of the hotels might have a discount booklet or a coupon booklet. I went with a school group, so we got a booklet of coupons for tons of places, such as the Empire State Building, Circle Line cruises, and many more.
A tip for the Empire State Building would be ready to wait in a very long and hot queue. If you have a fear of many people, this is not the city, and definitely not the building, for you. Get there early for a good place in line.
Best Way To Get Around:
New York is a city of walking people. Everybody walks everywhere because everything is usually a short distance from everything else. To go longer distances, you can take a bus, cab, or the subway. The subway isn't as bad as it is made out to be in the movies, so don't be paranoid about it. It's far cheaper than a taxi.