Thanks for a wealth of tips! We'll be staying in Budapest 3 or 4 nights, so may be able to make the 3-day rule.
The hydrofoil sounds intriguing, but paying out 180 Euros is a lot (especially after seeing how cheaply one can get the train tickets). Has anyone done it before?
We'll do some more research on Bratislava and determine whether we want to stay there. A positive for there is we have some free nights that can be used at the Crowne Plaza there which seems pretty nice.