One easy get a way spot for people in the GTA is Blue Mountain Collingwood. While many people just go there for the Skiing, it has become so much more over the years. I used to go there as a child, and what has happened to Blue Mountain, and Collingwood has changed it from a ski town to an all season getaway.
The hotels and chalets in the area have become more luxurious, and the major hotel chains have come to town to update the room rental space. This has brought more people to town and in most times the whole area is sold out.
In the summer months, there are beaches, trail walks, mountain climbing (I did this) water parks, and great dining experiences. There are also spas which offer salt springs which are usually sold out. There are also zip lines and small coasters through the mountain bottom. You can also go to the top of the mountain and ride down the ski lift and see phenomenal views.
In the winter, of course there is world class skiing, skating, and tobogganing.