In addition to places mentioned by Dave Min would recommend Belgrade and Skopje if you are in Balkans region.
I would 2nd Dave's recommendations on Ljubljana and Bratislava as places not to be missed.
I have been to Budapest and loved it and would say its worth a visit.
Brno in the Czech Republic also worth checking out.
I liked both Vienna and Zagreb although my view on Zagreb maybe down to the permanent drunken haze I was in on both my visits.