I went to these places out of your list.
Zagerb is a nice place to chill out, but not a huge amount to see and do...
I spent 4 nights in Budapest (arrived late and left very early) and could have done with 5 whole days to explore. Everything is amazing there and I very much want to go back.
Vienna I only spent 24 hours and spent a few hours finding the hostel that we booked and bailed as it was really far out of the centre and the route on the metro was closed for engineering works so would have made the journey about an hour long! We stayed in a hotel that was pretty much in the main square and didn't view much else. Not much about the place made me want to... I preferred Salzburg which was a much prettier place and had a better atmosphere.
Prague was OK, I preferred Amsterdam and Budapest (the places we saw either side of Amsterdam). Apparently we missed a few great places, the only one that comes to mind was a multi storey night club by the bridge (I think it's called Karlovy Lazne Club?)
Add Lake Bled to your list for a great chill spot with amazing scenery.