Another vote for Lisbon here. Small, charming city centre, good gay scene, great food/drink, hairy hunky men... not overwhelmed with tourists, so still has a lot of local character, the weather should still be pretty decent there but there are indoor things to do if necessary. Just make sure you stay around the Bairro Alto and not in the 'diplomatic district' where most of the hotels are otherwise you will spend all your time on public transport.

Barcelona's a must see if you haven't been there, but I think it's a bit unwieldy for a weekend if you haven't been there before - it's quite a big city and takes a bit of getting around. Prague's great fun and has a great scene and this is probably a decent time to go as it can get overwhelmed with tourists during the summer and over Christmas.

Also a big fan of Copenhagen - compact attractive city, good gay nightlife, hunky blond men, everyone speaks English, but it is expensive.

My suggested wildcard was also mentioned by joeinnewengland - Antwerp has a lot of buzz around it now as a real party centre and attractive city. It's on my list but haven't quite made it yet...