If you're staying near the airport, have the hotel van drop you off at a subway station if they will. From there you can get downtown for a few bucks. Just walking around downtown is a nice way to spend the day. Lot of little shops and cool cafes, bars, and restaurants. Or if you have a crew that's in the mood for it, check out the gentlemen's clubs, montreal is known for great ones.
For sightseeing, once you get downtown you can walk right up Mount Royal. That name was what gave Montreal its name; it provides a good view of the city from the balcony of some historic building. You can't miss the mountain from the downtown area. Just start walking towards it and you'll find your way up. I'd post a picture of the view but I don't have any on this computer.