I have no idea why I should go to Canada (maybe Niagara Falls, but I'm not sure if I would need to cross the US border?),
The Canadian side of Niagara Falls is what you want to see. Americans didn't invest a whole lot on their side of the river, so there is no point in crossing the border, unless you like Buffalo chicken wings (yummy)., or want to see what happens to cities when jobs are exported abroad. Once the 4th largest (in economic terms) city in the US, Buffalo is now known as a city with freaky snow weather.
The best time to visit Niagara is in Summer months when they offer a lot of festivals, special events and the area is simply pretty to just be here. Late Fall and Winters are drabby and not much to do or see here, even though they do some little tricks around the Falls.
When my sister visited a few years ago I kinda worried what I could show here here in the region, so I started doing research and long story short - after two weeks of intense tourism she siad enough, and just went with her hubby to Algonquin National Park for a couple days.
Toronto is just a stone throw away (100+ km) but frankly, other than a museum and an odd theater (like hundreds in Poland) I'm not sure why anyone would like to visit Toronto. Once an example of a great success in most urban studies curricula, it is now used as an example of a failure. A city without a soul, constantly trying to (re) define itself.
One "interesting" place in the area is Hamilton, about half way between Niagara and Toronto. It is called by some Ontario's Anus, as the only parts of the city visible from the highway traveled by millions are industrial installations.
As for the rest of Canada... when do I start? The biggest attraction here is raw nature. Pure and untouched in most parts. If you travel by private plane you can see quite a bit in two or three weeks ;)