Montréal is an amazing city. There are enough bars and restaurants to go to a different one every night for a few years, and that's a lot for a city of just under 2 million people. I lived there for three years and I miss it so much!
My favorite bar the last year I lived there was Baldwin's Barmacie (115 Avenue Laurier Ouest, no metro station). They have really great tunes and the best cocktails in the city, but it's a little dressy. If you want something more laid back there's La Distillerie on the corner of Rue Ontario Ouest and Rue Sanguinet (métro Berri-UQAM), where you can get cocktails by the jar-full. There's a great microbrewerie called Dieu du Ciel just down street from Barmacie (walking [north]-east) if you're more of a beer guy.
Gay venues that shouldn't be missed are Chez Mado, the funnest drag show in town (but not as much fun if you don't speak French, oh who am I kidding, Mado only speaks Québecois). Sky Bar and Parking are both very popular clubs (all métro Beaudry).
Get a smoked meat sandwich at Schwartz's, at 3895 Boulevard St Laurent just above Rue Saint Cuthbert (métro Saint Laurent or Sherbrooke, but if you get off at Saint Laurent you can take Bus 55 three stops and it's right there). Go for a poutine
at Patati Patata (4177 Boulevard St Laurent, three blocks north of Schwartz's) for a taste of real Québec cuisine (you'll be two blocks from my old apartment there, too
). Keeps your eyes peeled for Basha, a Lebanese restaurant with several branches in Montreal for really cheap lunch food and good falafel. The hot chocolate at Juliette et Chocolat induces oral orgasms. There's one on St Denis just north of the métro station Berri-UQAM.
Walk up to the observatory on Mont Royal to get a great view of downtown, and walk around the other side of the mountain to see a view of the other side of the city (and on a good day, the foothills of the Laurentian Mountains).
I just realized that you'll be there for Halloween, and Club Opera always has a Halloween party, but it's going to be crawling with first-year students from McGill and Concordia, so unless you're into that (and have a costume), steer clear. There'll be gay halloweenie things going on in the village, too.
I could write about Montreal all day, but I think there's enough stuff there to keep you occupied for a weekend. Some other people will definitely have some other things to suggest.
EDIT: AND SHOPPING, just go to Rue Ste Catherine, there are enough shops to keep you occupied for hours and hours.