Picnik electronic can is fun but I would go to Mont Royal. There are plenty of pubs and bars on Rue St. Denis. (Metro Mont Royal). If you don't enjoy some place, you can easily switch venues. Great to meet locals...
Weed is everywhere and fairly easy to get. You can smell it everywhere
Just ask people... No need to worry about cops either. They are lazy and don't care unless you screw up really bad...