Well let's see of you like sports than you are in luck! Toronto has a hockey team, the Toronto Maple Leafs which are always competitive in the National Hockey Leauge. Of course there is the Toronto Blue Jays and various other sporting attractions, but my Own person favroite is the Air Canada Center I have heard how amazing that place is, if you do not like sports you'll appreciate the outstanding architecture!
If your there during the NHL-season the choice is easy, go to a Toronto Maple Leafs hockey game and see the great Swede Mats Sundin. Otherwise you can watch the Toronto Blue-Jays if you like baseball.
If you're downtown, you should check out the Air Canada Center, home of the Toronto Raptors Basketball Team.