Chez Paré is better than Supersexe. Expect cold weather, its entirely possible that the temperature breaks the freezing point at night. As Goosemaster said, the Vieux Port is one of the major touristy places, but October is off-season so there won't be as much going on as if you had gone in June or July.
If you're into shopping, Ste-Catherine street has a lot of boutiques and shops, it has tons of small boutiques and a few worthwhile malls such as Les Ailes de la Mode.
St-Laurent is the major clubbing street, there are tons of various types of clubs/lounges.
St-Denis is the "French" terrace street, this is where you'll find Saint-Sulpice and a bunch of other pubs. Saint-Sulpice is nice but I guarantee you there won't be much terrace-ing going on in October. There are a bunch of cool brew pubs in that area if you're into that kind of thing.
Crescent is the more anglophone street for terraces, again it's kind of cold for terraces but its still nice. Another decent place is McKibbin's on Bishop, right above Ste-Catherine, it's a Irish pub on the first floor and it has a dance floor on the second, so you usually end up with people getting plastered and hitting the second floor. I believe Wednesdays is ladies' night, but I'm not entirely certain; Thursday is karaoke night, stay away from that, it blows in the worst of ways. On weekends, McKibbin's will usually have live music on the first floor. There's another place called Brutopia that's usually decent if you're into pubs, live music, pretty young crowd and reasonably good beer.
You'll probably want to walk down Prince-Arthur, it's near the McGill ghetto so it is very student populated. Prince-Arthur has a cobblestone section that is pedestrian only where there are a bunch of restaurants. Cafe Campus is a cool place to go out, they serve cheap beer which means that it often gets packed with McGill students.
If you can, you should try to get tickets to see the Canadiens play at the Bell Centre, it's really fun though a little spendy. Reasonably priced usually sell out pretty early so look it up asap. I think there's a link through
www.canadiens.com
There's also a really cool brew pub that serves interesting beer that's out of the downtown core called Dieu du Ciel. It's near Laurier metro station (IIRC it's at the corner of St-Laurent and Laurier). It's in the French part of town so it's pretty different from the rest, they serve these really interesting brews and the food is excellent. You're looking at a $5 pint for most beers, so it's pretty cheap to boot.
I think that should cover the basics of the going out stuff, if you're more of a culture type of person, there's plenty of information on the web so I won't bother covering it.
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