I've done Sziget in Budapest for 3 out of the last 4 years.
http://www.sziget.hu/festival_english
It's on an island in the middle of the Danube in August. It's usually roasty toasty, has a wide variety of acts, cheaps as chips and has the best festival food I've ever sampled by far. Plus you have the added factor of having Budapest to play with as well.
This year I'm off to Balaton Sound on Lake Balaton for 5 days in July. Staying in Siofok which is meant to be very lively.
http://www.sziget.hu/balatonsound