Things to do in Montreal
Montreal is a fantastic city. You may want to spend an extra day or two.
If you're a transit geek at all, you should probably allocate some time to spend in the metro.
http://stm.info/en (official website)
http://www.metrodemontreal.com/index-e.html (for transit geeks)
http://www.metrodemontreal.com/rating.html ("ranking" of the stations)
If you want to go to a museum, the Pointe-à-Callière museum near the old city comes very highly recommended. Its permanent exhibits cover the history of Montreal, and it contains the (in place) remains of many old buildings from the old city. Even if you're not the museum type, you will probably find this place to be extremely interesting.
For food, you should absolutely visit Schwartz's (3895 Saint Laurent Boulevard) for the most famous smoked meat on earth. There is often a long line to be seated at the restaurant, but they have a take-out immediately adjacent which has some limited seating inside as well. If all else fails, a reasonable alternative ("Main Deli") is almost directly across the street.
The main factory for Fairmount Bagel (74 Fairmount Ave West) has (arguably) the most famous bagels in the universe (having been on the international space station twice). Their best is plain sesame fresh out of the oven with nothing on it. There is nowhere to sit here other than some park benches on the street nearby.