Shady, you may know this, but some of the basics:
Old Montreal is beautiful, historic, quaint - you feel as if you are in an old midevil French city. The restaurants play off that, you have your touristy boutiques and some great arts and museums... plus a pretty cool public square at the northern edge that is like a grand blvd in Paris with bistro type dining on both sides and a feet active public space.
Rue St Laurent runs north from there and has traditionally been one of the centres of activity with a bunch of restaurants clubs a lounges. Bit upscale, mix of students (when in session) and the beautiful people. But the right kind of ugly counts as beauty in Montreal so you should be fine
Take that north and maybe a little east of there are some old school true neighbourhoods. Trying to find the name of a place where I had one of my greatest dishes (Rabbit two ways), in a modern but still feeling of country french restaurant. Too small and not appropriate for a Bach party though.
Rue Crescent is to your east and a major Center of clubs and bars... double edged sword as this is where you find your 18/19 and older clubs. Expect a bunch of drunk young American s... um new rules... sultry and wanting girls and women acting as such. There are a couple decent bars/lounges there also with patios And some decent Irish pubs as well. This bleeds into the neighbouring streets Rue Peel, Rue Stanley etc
St Catherine is the major binding agent and connective issues that runs east west from old Montreal and St Laurent to Crescent and that area. Tons of shopping from boutiques to a high end urban mall and everything in between.