If the citizenship of everyone allows it and the rental car agreement does also, from Niagara Falls, go to Montreal via Rochester and Watertown NY then come back over the border at Gananoque or Cornwall (cheaper gas by a lot) and when you get to Montreal decide if another 500 + km to Quebec makes sense based on what you've done and time left. Vs. doubling back from Niagara Falls to Toronto, and through Toronto to head east.

Overall it's too much driving and not enough sightseeing for such a short amount of time. The view of Lake Ontario is that it's a lake. You can see it from St. Catharines for 5 minutes and once you've seen it, you've seen it. While in Niagara not too far to see Lake Erie as well. Blindfold one person and point them at either lake and see if they can tell the difference. They won't be able to. They are just lakes. Big ones.