Amsterdam is the obvious party place, but certainly plenty of tourists. Helsinki is attractive--in winter I enjoyed the Art Nouveau architecture, and there are coastal sights to visit (haven't visited Talinn). Every Nordic country lets down its hair during the long summer nights, but the Finns will get out of town--lots of tourists on cruise stopover. Possibly many will be Swedes: the Baltic ferry from Stockholm is a major party because of punishingly high Swedish liquor taxes (food? what food?)

Budapest is great but I've been there for reasons other than partying so can't address that. It should be convenient by air.