I haven’t been everywhere in Canada so I can’t really say, but I can assure you that Montréal is very interesting, and it has fun things to do everywhere. (Not much greenery, but smaller towns in Québec, or the Laurentiens region where I currently live are fulla mountains and parks and rivers and crap.)
It’s quite a damn unique city, with all sorts of people everywhere, all sortsa crap to see and do and all. Check out old Montréal and the port if you can. Québec City is also supposed to be a sight, never been there myself but I’m sure it’s worth checking out.
You might wanna look up Vancouver if you’re at the other end of Canada. Another wicked big city, and there’s always some event going on down there. Never stayed there very long so I can’t really confirm, but it was awesome the two times I went anyways.
If you go to Manitoba, check out the city of Winnipeg. A good part of it is impoverished and full of crime, but it did something right with its many historical sites and places though. That city is steep in its Indian history. (The Forks, Fort Gilbraltar, the Exchange District. I’d mention Osborne Village too, but it’s complete Yuppie town now.)
Still I’d suggest Montréal as a first choice, it’s much more fun and diverse.
@dynamicduo has it right. It largely depends on the time of year you go…Winter is awesome in Québec and Manitoba for activities and festivals. Vancouver has the Winter Olympics. All these cities have just as much to offer in the Summer too. You might want to check out guides and websites for local activities and things.
@stranger_in_a_strange_land I totally have to go to Gaspésie some day. :D