The Sharbot Lake area is less than 300 miles from Niagara Falls. Since my interest is geology, I'd be headed there for another look at the metamorphic rocks in the Grenville Province of the Canadian Shield. Haven't crawled over those for about 40 years now.
A couple days in Toronto on the way there or on the way back, I love to visit Toronto and find it easy to explore on my own (using public transporation), like Chicago, NYC or most European capitals.
What are your interests? Where you should go depends on what you find interesting.
750 miles is a huge radius, even for this part of Canada. It will get you to most of the well populated parts of Ontario or Quebec, into the Maritimes (barely) or all the way to the north shore of Lake Superior, although some of the better routes to some places might be back through the U.S.