I'm not much of a tourist myself, but I'll give it a try.
Niagara Falls has the usual "touristy" things you'd expect, wax museums, floral diplays, scenic lookouts, etc. There are lots of golf courses in the area, the bulk of the Canadian river shore is parkland with many old forts, battlegrounds and other historical sites. Niagara-on-the-Lake is made up to look quaint with several good restaurants. There's also the annual "Shaw Festival" that puts on plays by George Bernard Shaw which may be running when you're there.
Toronto is something that people who like cities are really impressed by. The CN Tower is 1800 feet high and is a tourist destination. There is a nice walk along the inner harbour. The Skydome/Rogers Centre is near the CN Tower, and you may be able to catch a Jays game or the Argos (Canadian football is far more explosive and exciting than most of the fare you'll catch south of the border.). The Skydome is near where Zebulon Pike (of Pikes Peak fame) was killed during the War of 1812. The Hockey Hall Of Fame is a short distance from the Skydome as well.
The theatre district is one of the best in the world with many fine restaurants in the area. The Canadian National Exhibition may be on when you're there, which is a big fair/agicultural exhibition/manufacturers show.
Montreal is a very "cosmopolitan" city with its french culture. Again, dining is first rate, the women are beautiful, and you may be able to catch a CFL game. There are several historical sites, but I can't recall them now.
If you decide to swing by Ottawa, we have the Parliament Buildings, many parks, museums and scenic places. The bullet riddled cockpit of Billy Barkers WWI plane is on display at the War Museum. I understand the National Air Museum is very impressive as well.
If you continue down river from Montreal, you get to Quebec City which is quite a tourist attraction as well with its walls and colonial style buildings in the old city (citadel).
