Art Gallery of Ontario
Was there a few years ago. Mostly artwork upstairs, large ship models downstairs. A bunch of various other exhibits.
Ontario Science Centre
CN Tower/Restaurant
Ripley's Aquarium of Canada
Niagara Falls
Huge waterfall and tourist trap about 130km from Toronto. I've been there once before. October is a bit late in the season for watching idiots attempting to survive going over the falls but will also be much less crowded. Could rent a passenger van in Toronto (10-12 people) for $200-$250 for a 6-hour day trip. Anyone with a valid USA or Canada driver's license can drive it.