You'll have a ball. It's a really fun place. Make sure you get a visit to the Mountain Equipment Co-op shop if you're keen on kit!
The Ontario Science Centre is worth a visit, and Canada's Wonderland is great fun if you have kids. Check out the CN Tower if you enjoy vertigo, and St. Lawrence Market is good for a browse, as is the waterfront area.
Enjoy the street food as well - there's plenty of good scram to be had from the street vendors (use common sense, obviously!).
Oh - and make sure you do the Niagra thing. It's properly awesome.
Have fun!