I live in southern Ontario, and have fair knowledge of what's in my backyard. Northern Ontario is a different story.
You will discover many excellent touring opportunities in southern Ontario. I have based many trips on the Waterfront Trail, but there are other possibilities, including hybrid trips. On one trip, I followed the Waterfront Trail from Toronto to Kingston, and then zigzagged north-east along (mostly) quiet roads and highways to Ottawa. I have also gone in the other direction, toward Niagara Falls. The backroads that crisscross the Niagara Escarpment are not part of the Waterfront Trail, but are great.
I have toured in Prince Edward County many times since the early-1990s. The absolute best touring experience I had was in France, but Prince Edward County comes a close second. I can't get enough of it. I returned to the County three times in 2009. There is always something new to discover.
Areas of Ontario that I have visited but have yet to tour by bike: backroads near Kitchener; Guelph; Milton; and Peterborough. Following the Ottawa River from Ottawa and Petawawa looks interesting. Next year, I am thinking of tracing the north shore of Lake Erie.