Go to LaFleurs for your poutine fix, it beats the shit out of La Belle Province's poutine any day of the week. There's one just above the corner of St-Denis and Sherbrooke, right next to this park with a fountain called Parc St-Louis (which is one of Montreal's finest outdoor drinking spots, BTW). You won't regret it. I took the Catholic Boys there a couple of years ago and they were all like "this is the best shit I've ever eaten in my life."
Other suggestions:
Avoid St-Laurent street below Pine avenue at all costs if you can this weekend. The F-1 Grand Prix is in town this weekend and every single rich fucking asshole in the universe with Ferrari's, spinning rim Escalades and customized Honda Civic's will be partying on the lower part of that street. Everything on St-Laurent above Pine is fine, though, and much recommended to visit (since most of the cool bars and show venues are on that part of the street). Check out Miami, Copacabanna and Barfly (the best hole in the wall bar in the world) for fun drinking times.
Also on St-Laurent above Pine: You absolutely must try a Smoked Meat sandwich at Schwartz (which will be packed, you've been warned). It blows away any meat sandwich you've ever had. Go around 3 AM or in the middle of the afternoon if you don't want to wait in line. And dig on the $2 Chow Mein at this take out Thai place on the corner of Pine and St-Laurent (can't miss it, its got a take out window).
The drum circle thing that Josh is talking about is this lame hippie fest known as the "tam tams" where literally thousands of people gather on Parc Avenue just beside Mount-Royal on Sunday afternoons to smoke weed, play hacki sack and (if you're into Live Action Role Playing) beat each other up with plastic swords and axes. Its fucking retarded, but its very unique representation of the kind of street hippie phenomenon that exists in Quebec.
Other cool spots:
Olympic Stadium: far from downtown, but its one of the most amazing pieces of architecture you'll ever see in your life. Its like a massive spaceship with the worlds largest inclining tower jutting up from it. Too bad it was pretty much the worst sporting facility ever once you got on the inside.
Au Dieu Ciel: Amazing bar on the corner of Laurier and St-Laurent. Best homebrew I've ever had, hands down.
St-Denis street: great street to walk up and down in the summertime. Lots of great stores to take the wife to, as well. Some of the bars are a little on the yuppie side, though, but pretty much all of the best record stores in town are also either on this street or very close to it.
Mount-Royal: The tam tams are fucking stupid, but you should still check out the mountain if you can. There's a giant lit up crucifix on the top of it and lots of beautiful parks all around it.
I don't feel like typing anymore, so maybe Alex C. will fill you in on more shit to check out.