Stag do's

[Dutch Uncle]

Just to throw a few more names in:

Maastricht is a beautiful place and has more pubs/bars per 1000 population than anywhere else in Europe. Just beware of Carnival about 6 weeks before Easter. Other Dutch towns if you don't fancy Amsterdam would be the University towns of Leiden and Delft.

For German places, apart from Hamburg which is great, I always liked Aachen, Koblenz, Trier and Munich (expensive) especially around Oktoberfest - which is of course in September because of the change from Julian to Gregorian Calendar in 1583.

[Dagenham Rover]

Prefer Maifests myself not quite as busy and touristy