Frankly, you easily could spend the entire time in Montreal and it would be less hassle as well. There is quite a bit to do and you won't get bored.

However, if you prefer to check out Mont Tremblant, either for a future vacation or just a change of pace on this trip, it certainly is doable. In fact, many people in and around Montreal go to Tremblant for day trips, winter and summer. If you are flexible and do have an interest in Mont Tremblant, you might want to make a last minute decision, depending upon the weather forecast and your mood. Montreal offers much more in the rain than does Tremblant.

DRIVING TO MONT TREMBLANT. The roads are very good and the drive from downtown Montreal is a fairly consistent 90 minutes (and can be less), depending upon time of day, traffic, etc. So, it is certainly possible to leave Montreal at 9:30 Sunday morning and be at Tremblant comfortably by 11:00 a.m. If you want to stroll around, go to the pool, ride the goldola to the summit, hike or bike on Sunday afternoon/evening, it is perfectly doable.

If your flight home is late Monday afternoon, you would have to leave Tremblant about four hours before flight time. It will take 90 minutes from Tremblant to Montreal's Trudeau International Airport, plus you will need time to gas and turn in your rental car, check in for your flight, go through the US customs and immigration pre-clearance (which can take a little time), etc. I usually accomplish all of this in 3 hours, but allow an hour's extra time in case of road construction or traffic delays.