Mississauga River
The Mississauga River (no where near Mississauga) is a great little weekend run near Peterborough Ontario in the Kawartha Highlands Provincial Park. It's a nice tight river with a bunch of fun whitewater. Some of it is pretty big though and if you're not running those, it's quite a bit of portaging—the kind where you can see the next portage while you're loading your boat at the end of another portage.
One caveat though, is that it can be difficult to get a sense of the water levels before going and it can run dry. It may be a good idea to pop over to Canadian Canoe Routes forum and ask about water levels in a post before heading out. It's not as seasonably predictable since it's dam controlled so local knowledge is key.