I've toured around the Kawartha Lakes area and can recommend it. Take public transportation to Peterborough and then ride back to Toronto via Bobcaygeon, Fenelon Falls, and Port Perry. It's a great ride. There's a trail connecting Fenelon Falls and Lindsay, great for stealth camping.
You might also consider Loyalist Parkway in Prince Edward County. It's possible to ride from Kingston to Toronto in two days via the Waterfront Trail. Take the ferry at Glenora. Avoid the Murray Canal service road.
The west side of Lake Simcoe is great, but Highway 12 east of the Lake is highly dangerous. A few months after I did a loop Lake Simcoe ride out of Toronto, a cyclist was killed on that road. I recommend a parallel road.