Definitely seek out Ted's Range Road Diner. It is East out of Owen Sound going towards Meaford.
Rockford Restaurant is also good - You will have to pass it on your way up 6/10 just South of Owen Sound. It is in Rockford of course on the corner.
Also - Check out South Hampton beach for a quiet Sauble type beach.
I am looking for real diner type places North of Toronto from about Hwy 9 to Barrie , Orangeville to Innisfil that serve more than just burgers.