You might want to start keeping an eye on Groupon/Living Social to see if anything pops up that you'd like to do. It seems to be often hair salons and mani/pedis, but sometimes fun things to do or restaurants to try pop up (and maybe you get get a mani/pedi!) Sometimes hotels are there, too.. I think?
I'm sure you have plenty of ideas of things to do or see, but if you'd like more, TimeOut New York tends to have pretty good event listings (sometimes cheap-to-free).
And I'll second the airbnb idea. I have a friend who swears by it as a way of finding lodging at an affordable price, and another who lists a room on there and loves it (I'd recommend her, but she prefers longer-term guests). However, you should look at booking those *now*.