One thing you should keep in mind though; The dollar has rarely been as strong at anytime as it is now compared to the different Scandinavian currencies, som now may be the time!
Being from Oslo I do second the fact that we do know how to party and some of the hotels might even still be offering Summer Deals in the earliest part of September. The temperature situation, though, might be dodgy.
If you really want hot weather I second austman's suggestion of BCN. To me: The motst complete partycity ever if you know where to go or rather not to go. Lots of sights, actually friendly people ++. Fantastic cooking and probably the best value for money anywhere in the "civilized" world(?).
I'm going back in March and twice in November and my first shot at a trip report may be from the one in March.