Agree with Dave's recommendation, Algonquin's Canisbay is a bit back from Hwy 60, quiet and woodsy. Stays open till Canadian Thanksgiving. One area is Radio and Pet free, so watch for that. I have 6 nights booked late Sept, then I move to 6 nights at Two Rivers. There I have a water access site so I can leave the kayaks at the shore just a few feet away.
Algonquin gets very busy with "foilage" day visitors (can be crazy on weekends) from mid Sept till mid Oct. Some lodges in the park are open for meals and that can be a great option for a change.
If you like to hike or paddle it's hard to do better than Algonquin.
For hot weather Bon Echo has great beaches and some paddling in open water. Hiking is OK but just a few trails. Also, Charleston Lake has good beaches and very good paddling, but hiking is limited.
When booking at Ont PP's check the Site details. Most will show a distance to the electrical post (in meters) and give an equipment size statement (in feet??), like Trailer up to 25 ft.
Fred, Gail and Sandee the Puggle
2008 Pathfinder, Native Ultimate 14.5, Delta 10
2012 Rockwood 2503S