Definitely hit Niagara Falls and make your way around the golden horseshoe to Toronto. T.O. is hands down my favorite city in the world. Think New York City, but cleaner, friendlier, safer, more vibrant, more cosmopolitan and just all around better. It also has the best downtown I've ever had the pleasure of experiencing.
I'd skip Montreal. In my opinion the city is VERY underwhelming and FAR too overhyped. It is nowhere near as "French" as people like to make it out to be. Honestly, it felt like an American city in every single way save for the French street signs. I was quite disappointed.
If you're planning on visiting a second Canadian city on the east coast, I'd recommend Ottawa over anything else. Beautiful capital with a rich history. I preferred it over Washington DC, although I'd highly suggest you hit BOTH capital cities if you have time.
On the west coast, San Francisco is a must. It's much bigger than you think and has some amazing street life and great architecture.