September in Muskoka is often warm days, as high as 80F and cool nights (50F or cooler).
Also, beware that if you rent a car, all of Canada is on horrible metric, which I've become used to (reluctantly). All highway speed limits and distance signs are metric---4-lane highways typically 100 kmph (kilometers per hour), about 60 mph. Most traffic moves at close to 120 k or "clicks" as we Canadians say, which police usually allow (about 72 mph). I've never been stopped driving 118 km on 4-lane roads. Two-lane roads are set at 80 kmph.
The speedometers on rental cars will be metric with a smaller inner circle in mph that's often hard to read. I use a portable GPS in my car that I can switch from km to mph, handy for US/Canada driving.
Temperatures on radio weather forecasts are mostly in metric, 20C being pleasant, about 70F. Quick conversion is to double the metric temp and add 30. (20C = x2, +30 = 70F.)
Weights in supermarkets are posted in price per pound and in kilograms, with good ol' pounds being prominently displayed.
Hope you can make it. Toronto's airport is Pearson, (YYZ).