Fun things to do NYC

So for my bfs bday (he turns 21 I'm still 20). We are going to NYC in December. We will see the Christmas tree, Bryant park and maybe ice skating if I can convince him. As his gift I eyed to get tickets to do something. He doesn't want to see a play/musical so that's out of the question. I would prefer to do something that isn't at a set time. Any ideas? We will go out for a nice dinner after with his parents but I want Ed something special for us to do! Any ideas?