Me and my buddies did it a few years before turning 21...it's a good trip only if youre undeage in the states..my suggestion would be to get on a boston->montreal "party" bus for cheap. There are a million package options that will usually combine round trip bus ticket + hotel (fit a fuck ton of ppl in there and divide the cost) plus an incentive to go to a few clubs/bars on st. catherine street
Once you get there just drink your face off and wander in an out of titty bars and various clubs on the verge of black out