Quite populr with stag doos - we're off there for my brother's stag doo - fortunately we do have a few older blokes going so it won't be a mad booze up - certainly want to do some of the museums between pub stops. I'm trying to persuade them to do the beer bike !!
I can recommend The Hospital in the Rock. It's set in the caves within the rock which the castle is on. Used as a hospital in WW2, and in the 56 uprising, then as a Cold War bunker. You have to go on a guided tour (which is in English), and there are several areas set up as operating theatres etc with very good wax works and other displays. They even let you wind up the air raid siren!
The Secret Service museum is also very good, if a bit sobering.
Some of the other museums only had information on the displays in Hungarian (the Army museum springs to mind for this) but were very good nevertheless. Loved being able to handle a PPSh41.
When I went there were 400 Forints to the pound, so 10000 Forint notes aren't uncommon, which makes you feel very wealthy!