Orlando to Toronto

95N through to New Jersey then the NYS Thruway to Syracuse then 81 north to the Canadian 401.
Not a large mountain on the route.
or 95N to Washington area then US17 to I66 to to I81 to Canada then 401 to Toronto.
More mountains than first route

While the above will certainly get you there, it's going to be lots of cities (read traffic) and not the shortest flat route either.

If you take I-75 north and onto I-24 to Nashville, then onto I-65 north to Lousville and onto I-71 to I-75 to Cincinnati and on to Detroit or Port Huron it's a very flat drive with no real mountains, just some hills.

I have gone back and forth many times between the Toronto area and central Florida. My preferred route when towing is I-75 all the way to Detroit, then Ontario Hwys. 401 & 403 to home. Generally speaking the shortest way is I-95, I-26, I-77, I-79, I-90, Queen Elizabeth Way. It's about 100 km. (62 miles) shorter than the I-75 route. Even though it is a little longer on I-75 it takes about the same amount of time and I find it a much easier drive when towing. Just avoid rush hour at Atlanta and Cincinnati and you will be ok. The only real stretch that has any hills of significance is the 15 or so miles at the north end of Tennessee.