If you're looking for a place to drink, eat and just sit back and people-watch, then the patio on Crescent are the right place. This street is nearly always busy. It has some good pubs (Wiston Churchill) and good restaurants (Pino).
If you're looking for nightlife, this is where the tourists hang out. There are a lot of college age people making rounds at the various bars and clubs.
A must see!
TIP: parking can be difficult. Try to look for reasonably priced parking on Ste-Catherine (away from the busiest part). There is usually parking on the street close by on Ste-Catherine (you can pay at the meter and update the meter if you decide to stay longer by using your phone -- just remember your parking number). It's only a few minutes to Crescent.