Ha! Well there is no shortage of food, shopping or people watching there...that is for sure. We do a TON of walking when we are there. Do you know where/what area you are staying in yet?
Central Park is a must. It's huge so you should get a map of it b/c there is so much to see! It is very cool to just walk around. A lot of times on the weekends, they have craft fairs, concerts, or something else going on.
Times Square. You can stand in line at TKTS and get discounted show tickets. Definitely worth it....if you like that sort of thing. I'd read up on what is currently playing and see what sounds good.
Empire State Building - I actually don't even remember if I've ever been to the top, but it seems like it would be a really fun/cool thing to do.
Ellis Island/Statue of Liberty - again, I don't even remember ever doing this, but it's because when I go I don't really do touristy stuff b/c I'm always with my brother or my aunt/uncle/cousins.....
FAO Schwartz!! You'd be surprised how many adults are there without their kids. ha! Speaking of toy stores - the Toys R Us in Times Square is pretty freakin' cool too.
Rockefeller Center - they have the NBC tour there, which is neat. Ok, I guess I have done some touristy things.
Also the South Street Seaport looks really cool. I haven't been since I was little and keep wanting to go there every time we are in NYC, but it seems like we always go when it's cold and/or just don't have time.
Oh and seriously one of my favorite things is the Museum of Natural History. MOMA (Museum of Modern Art) is good too.