Brussels? Not many places with a better beer scene - breweries aplenty (especially if you're into sours, as Cantillon is unmissable).
Only an hour's flight away - shouldn't be much more than a hundred quid return with Aer Lingus, or cheaper to the other 'Brussels' airport with Ryanair.
Prices for beer are pretty decent (especially in the supermarkets...hello Rochefort 10 for 1 euro 20!), although food and accommodation can be pricey (and often shite in my experience).
The city is a bit drab, but it's class for a three-dayer on the beer as I imagine your stag do would be!