Aww hell... glad to see that you are in almost the same boat as me. Me and a few friends were considering going to the US last summer but since none of us was 21 a lot of the things we wanted to do weren't possible or let's just say involved "difficultys" (major headache: renting a car)... so we just went to spain (Ibiza) instead, got massively drunk went to alot of clubs and just had a two week long party
recommend it to anybody, just got to put up with the Germans and Brits who dominate the place

If you like it quieter and with a bit of culture (u still can party very well though) I recommend Prague.
Other european citys:
London is way to expensive (realize it everytime I go to visit family) AVOID RIP-OFF BRITAIN
Rome is great fun (their style of driving is something else... THEY AIM RIGHT AT U!! it's GTA3 in real life)
Barcelona is also high on my recommended list, very nice place!