Originally Posted by Charlatan

Algonquin is more than 2.5 hours and isn't in the GTA... It is a great place to go though.

The Provincial Park at Sibbald's Point is OK... It is on the southern tip of Lake Simcoe and has a nice beach. That said, it is a provincial park.

There are probably some nice places to camp and hike along the Bruce Trail... those would be reasonably close.