Props to poutine. No sober person in the world eats it but it seems to make such perfect sense after a dozen Fin Du Monde or Maudites.
I go to Montreal two to three times per year. This time of year, I would strongly recommend that you start on Rue Crescent because a) it's mostly English speaking b) tons of bars with patios c) close to McGill Univ so younger crowd. My favorite place there are two Irish style pubs called Hurleys and McKibbons. McKibbons has a great backyard patio and I've always found that the "scenery" is worth watching. There are some more dance club kinda places like Thursdays. Weird s**t has happened to me there. I don't want to talk about it.
As you go east from the Crescent St area, you should walk along St Catherines St. As everyone has said, there are tons of establishments featuring graduates of the Montreal School of Interpretive Dance. Stop in at the Peel Pub if you want to fill up your stomach with cheap eats and beers. It's an institution. And I promise you that they no longer recycle the half used draft jugs. Really!
If you want to hang out with more of the locals and French speaking crowd, I would recommend you go towards St Laurent Blvd. On Prince Arthur is a great little bar called Vol de Nuit. I like to head there before going to the local concert halls.
But to me, the ultimate summertime bar in Montreal is a place called Bar St-Sulpice, which is on St Denis. It's in the middle of a very cool party neighbourhood and is possibly one of the largest bars in Montreal. It's got four floors with connecting terraces and a HUGE backyard patio. You can't beat it, especially just after classes are done for the day at nearby UQAM.
By the way - all the places I've mentioned are within a 10 minute walk of each other or a very very short cab ride.
Have fun there this weekend! I'll be going next weekend to see Sarah Harmer on Canada Day and then Feist the next night in Quebec City.