Mara Provincial Park (slideshow above. Click on photo) located along the shores of Lake Simcoe and south of Orillia is our token beach getaway. It's about a 2-hr drive from Toronto and easily accessible. We've been here over five times leaving early in the morning (about 6-7 AM) and arrive around 10. The beach is usually sparsely populated and you get to pick whichever spot you want.
The waters are pretty cold but you'll get used to it. Beach has fine white sand and there's volleyball nets for anyone who's into that. Favourite has to be the floating island a few yards from the shore. Entrance fee is $15 per person.
Only downside is having to drive out to town to pick up supplies but hey. That's why it's a getaway ;)