Belgrade has one of the best Cat. 4 properties in Europe, so the two are fully compatible ;)

And frankly, although I may be a little biased as a resident of Belgrade, I can't think of a better (or better-value) place for a stag do.

You'll have to do without a beach, but food and nightlife are divine.

And what about Prague or Bratislava? They seem to be quite popular now. I had mine in Bratislava. Bit on the quite side but still fun and affordable, also plenty things to do apart from the obvious drinking and partying. We tried some target shooting and it was great fun, no complains from anyone. Definitely recommend this place http://www.bratislavashootingclub.com/