+1 for Gibby's
Not yet mentioned is Stogies (around the corner from the LCDH). A great place to grab a drink in the afternoon and watch the world go by while you have a cigar.
For nightlife, pick any bar on Crescent St. I happen to like Thursday's quite a bit. After the bar (
) try to hit a place called "La Belle Province" for authentic Montreal steamed hot dogs & poutine.
The Old Port is absolutely worth seeing, make a point to lunch at a bistro with a nice patio for the people watching.
You should PM Barteur (Eric) as he's a local and can give you more up to date info as I moved from Montreal 15 years ago (although I was there for a Stag party 2 weeks ago
)
What hotel are you staying at (might help in giving more relevant advice based on your location)