I would love Barcelona, but my fear is the others would think it's too expensive. The W Barca that weekend is €256 per night - fine for me, but not everyone I guess. And no, it needn't be a beach stag, I don't think.
I was looking at the W Istanbul, which I can get for €170. If some of them will share rooms, that may be much more palatable!
Though the nightlife isn't for me, I'm sure it is for the others, so it's got to be somewhere with places to go (exactly like Barca). On paper, the W Istanbul is starting to look really good. However, I've never been there, so don't know what else there is to do around the hotel (will do some research). Going to look into Casablanca too - the pics on spg.com look good at least!
Thanks for the suggestions all!