I'm afraid you might have to push your 2.5h driving limit up to 3-4hrs for the best secluded sites with a lake.
Our favourite Provincial Parks are Cypress Lake, Rondeau, Sandbanks and Bon Echo.
At Bon Echo, in addition to the main lake/beach, you can hike 3km or canoe into the empty and secluded beach on Joe Perry Lake. That's where I proposed to my wife.
Check out the Ontario parks website (
http://www.ontarioparks.com) for more details.
It's a very well designed site with lots of info and you check availability and book online.
Have Fun!