We had a two week holiday to Canada in August a couple of years back. It was absolutely amazing, and would happily do the same trip all over again.
We went to Toronto for the first 4 days, we stayed in an Air BnB in one of the suburbs a little way out of the city. Public transport great, no need for a car at that point. We did a trip to Niagara Falls, stopping at Niagara-on-the-Lake, this was an organised bus trip, nice and easy.
We then got our hire car and drove north of Toronto to the Muskoka lakes region. We had a cabin in the woods (also Air BnB), we stayed for a week there, and drove round visiting lots of beautiful little towns, swimming in the lakes, visiting farmers markets, museums, parks, etc.
And finally we drove on to a town called Picton, which is in an area called Prince Edward County (not to be confused with Prince Edward Island). Again, stayed in an Air BnB, we explored some more around there, including spending a day at Sandbanks National Park, which is on the shore of Lake Ontario, and has beautiful sandy beaches, it really does feel like you are at the coast. So lovely, and really not particularly busy.
We drove back to the airport from there (about 3 hours if I remember correctly), breaking up the journey by visiting Toronto zoo (late flight).
We did this trip with a young child, but it would be perfect for teens as well. Loads to do in Toronto, we could have spent longer there. And the scenery once you get out of the city is beautiful, and you can do all sorts of outdoor activities. The weather was also amazing for the majority of the time we were there.