Get a car.
If you plan on seeing more of Toronto than downtown and its surrounding neighbourhoods - and if you have a week to kill, you certainly will exhaust downtown in about two and a half days - I would strongly advise renting a car. Some of the most interesting sights in this part of the world almost require having 4 wheels: Pacific Mall, the McMhichael Art Collection, Niagara Falls, Muskoka, etc, while some of Toronto's most impressive sights are actually roads themselves, like the enormous 401, which could very well be the busiest freeway in the world.
If I were a European tourist, some of the things that Toronto has that would blow me away include one of the world's largest freshwater lakes. Go to the Beach(es) neighbourhood and contemplate how a body of water that seems to go out to infinity just licks at the shore like a duck pond - no influence of tides or storms, here. To compare, Germany's largest lake, the Bodensee, could fit twenty times over in the space of Lake Ontario - and Lake Ontario is hardly the most impressive of the great lakes, Lake Huron (and if you can make it, Lake Superior) are far larger. Grand Bend and Wasaga Beach are nice little beach towns, except you have to remind yourself that you are on a lake, not an ocean.
Toronto's ethnic diversity is best seen in the suburbs. There's the new Hindu temple at Rexdale and the 427 and the giant Pacific Mall is one of the main shopping destinations for the half a million Chinese people who call Greater Toronto home. Also, there are a couple of malls around the Highway 7/Leslie area in suburban Richmond Hill that are totally devoted to the Chinese emigrant community. Inside you will find restaurants serving some of the tastiest food that puts the Chinese food of Europe to shame.
Niagara Falls is almost best seen for its tourist traps. There's a street called Clifton Hill which makes Las Vegas look downright cultured. Not to be missed!Speaking about cullture in Niagara, take your passport and go to Buffalo and visit the absolutely phenomenal Albright-Knox museum. If you like contemporary art, this museum is the most underrated gallery in the world. Yes, world.
Get yourself a good map. There are a lot of streets and road signage in Ontario is furchtbar.
And don't forget to rent that car.