lol, it's funny, this seems like such an easy question for someone like me who's living in toronto all my life, yet when i have to answer it it's a bit a challenge.
This city has a TONNE to do. I'll admit I haven't experienced most of it lol, but I know it's out there. One really good resource is www.toronto.com which has a tonne of info on almost anything you can do here. The city's official tourism website can be found here http://www.seetorontonow.com. I don't use it as much (I'm not exactly a tourist i guess), but it's similar to toronto.com in that it gives you a crapload of info on events, attractions, festivals, and so on. They even have a "Gay Toronto" tab which I find really interesting. I don't really know what kind of things you like, but surely you can find something that interests you on those sites.
Some of the obvious places have already been mentioned: CN Tower, Royal Ontario Museum (ROM), Art Gallery of Ontario (AGO), Rogers Center (formerly known as the SkyDome), maybe catch a show at the Air Canada Centre or Roy Thomson Hall, visit Canada's Wonderland, etc. etc.
A few perhaps less-advertised things that I personally enjoy is spending some time along Toronto's waterfront. It's really quite beautiful (with beautiful people too, hehe). Woodbine Beach is probably the busiest stretch of the beach, and it's got a 50m Olympic-sized swimming pool, along with tennis courts and volleyball nets to boot.
I also love indulging in Toronto's many neighbourhoods. Some that jump out at me include The Annex, College Street, Little Italy, Yorkville, the Financial District, Yonge-Eglinton, Kensington Market, St. Lawrence, The Distillery District, Cabbagetown, Chinatown, etc etc. I guess it depends where you're staying too, as some of these will be more accessible than others.
Finally, you have to visit Toronto's Church-Wellesley Gay Village. It's a hub of anything gay and boasts some decent restaurants, fun nightclubs, bars, and, of course, cute gay boys.
If you have more specific interests or things you wanna see (ie. Shopping, Parks, Recreation, History, Arts/Culture, Music, Fun stuff, Gay stuff), shoot me a message or post here.
Telling someone about what there is to do here requires a 300 page book, and can hardly be done in a single post. But here's my shot at it.