If you weren't going in December I would tell you to go to the Toronto Zoo. But it'll probably be a bit too cold :rofling:
It's been since high school since I've spent any amount of time there, but definitely stay downtown - Sara's right, you'll save money in the long run b/c you won't have to park or drive anywhere. I think I stayed at a nice Quality Inn that was on the subway line, or very close to it. And we were all high school students, so I don't think it was very expensive...well at least not in 1995.
Edited to say: CONGRATULATIONS!!!