Having visited Montreal last Fall, I'd definitely recommend old Montreal.
Other things of interest:
The Olympic Park/BioDome (No, not based on a Paulie Shore movie).
Hike up Mont Real...it has some great views of the city.
If you've never been to a Moevenpick restaurant, try it out...it's pricey, but pretty cool.
Take a boat cruise up the St.Laurence. I had my doubts, but it's a really cool way of seeing the city from an entirely different angle.
If you know French, it will be infinitely easier getting around town. Anyone trying to determine Ouest/Est will know what I mean.
Do NOT try Poutine! It is one of the most vile substances every concocted. I couldn't get past the smell. However, Steak Frites is your friend when you can't decide on which French entree you want to try your luck with.
My overall advice is to make sure you don't miss Toronto when in that part of Canada. That place rocks.