We found that it was cheaper to not bother with the Inter-rail pass, and just bought trains.
In three and a half weeks, we visited Slovenia (Ljubljana and the Julian Alps), Hungary (Budapest and Balaton), Slovakia (just Bratislava) and the Czech Republic.
I’d have liked to see the Tatra mountains in Slovakia, and get across Cracow in Poland, but there wasn’t time. (I didn’t see the point of going to somewhere just for a day, just to ‘say’ you visited it…)
A tip for anyone wanting to do a similar thing: Buy local (national) trains wherever you can. There is a killer saving to be made. For example, it cost us next to nothing to get from Budapest to Bratislava, by buying tickets from Budapest to Esztergom, then crossing the Danube bridge on foot (and therefore the border), and buying another (national) ticket from Sturovo to Bratislava. A quarter of the price of the direct ticket! (and if national trains aren’t an option, buy a ticket to the first place over the border, and then another from there) 7 years ago