I suggest you ride around Lakes Huron and Michigan. Starting from Toronto, I have ridden around the three nearest Great Lakes in three days each. I can manage about 1000 km per day, but I was riding with a fellow who was only 66, so we kept our daily distance closer to 700 kilometers.
By the time you reach Sarnia, you will have an idea how far you like to ride in a day, and if circling two lakes seems like too much work, you can just go around Huron. The coastal highways in Michigan are very pleasant riding. I haven't explored Wisconsin in a long time, but I recall it was pretty nice.
But do this ride clockwise, so that you are nearing home when you reach Manitoulin Island. There is a lot to see on Manitoulin, and if you are ahead of schedule, you can explore the island thoroughly. If you are low on time, you can leave Manitoulin for next time.
In any case, here is your ride back to Tobermory, as you likely know already:
Welcome to the MS Chi-Cheemaun - Tobermory Ferry to Manitoulin Island
Ya know, this sounds like so much fun, I might tag along with you ...
PS The circuit around Lakes Huron and Michigan is not much more than 2200 kilometers, so you can add lots of side trips in case you want to visit Bad Axe or see both Sheboygan and Cheboygan, or even tour the thriving metropolis of Petrolia ON.