If time is a constraint, check Mont St Hilaire
http://www.centrenature.qc.ca/en/activities/index.html
or
http://www.mcgill.ca/gault/access/
If you do go there please get the trail map so that you can choose the different paths that you will be comfortable with . They are mostly easy trails .
15 yrs ago Tremblant was OK for a short sunday hike but unless you go on the other side ( which I haven't got to) it is a waste of time. Since it is September, you will spend at least 2hrs just to get to Tremblant ( may be more) there from downtown and then you have to do your hike and then back to the city again.