Madame Tussaud's Wax Museum on 42nd St is fantastic. I always recommend it to people.
Go to the Empire State Building for the view. It's beautiful on a clear day.
Take the Staten Island Ferry (free) past the Statue of Liberty from South Street Seaport.
Go to Chinatown, Little Italy or the Village for lunch. You're sure to find a cute little themed restaurant that you won't find anywhere else.
If you're into museums---Museum of Natural History and the Met are outstanding. Both will take hours to go to. Also, the Cloisters (medieval history museum) is very different and weird to see in Manhattan. You get a nice view of Manhattan on the way up since it's so far north.
Go to 5th Ave and window shop for all the clothes and things you could never afford in this lifetime
Hope some of these help. If you have anything specific in mind, let us know and i'll try to think of something. have fun!