If you had more time I would suggest window shopping down 5th Ave or Madison. I'm a big fan of gawking at the $$$ stuff I can't afford. But then I go do my most favorite NYC thing...a good mani/pedi!!
I agree with Cream. Go see at least one exibit at a museum. And agreeing with Brandon, go see the Empire State Building. If it's a clear day you'll get a great up above it all view of all of the city. Nice photo op. too.
But beware the lines!!!
If you're going to go somewhere with a line to get in, choose only 2 for the day. Not only does it save time but it will probably save your sanity as well. Lines blech.