In need of some inter-railing suggestions

I'm going inter railing this June with a friend and we have a three gap in our plans. We're starting with five days in Berlin then three days in Prague and three days in Budapest then we have a three day gap before we spend two nights in Bled followed by two nights in Ljubljana. I was wondering if anyone could recommend somewhere to spend our three day gap? My friend and I are both 21 and we're students so we're looking for somewhere cheap enough to stay with interesting new thing to do and to see and somewhere with good nightlife. It would also need to be accessible from Budapest by train in reasonable time and allow us travel to Ljubljana in reasonable time (i.e. less than 24 hours as out travel pass only gives us five days travel time). Also if anyone has an tips for students staying in any of these cities and looking to do things on the cheap but still have fun please let me know

Thanks!!