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).
Axiom Resident Expert