Well, I can honestly recommend everywhere I've been in E. Europe! Poland, Hungary, Bulgaria and the Baltics were all great.
Poland: the trains are great to get around on, and the people are friendly, I thought. Lots of neat historical sights like Krakow and Malbork (Teutonic Knight's castle).
Hungary: Perhaps not as tourist-friendly as Prague, nor as large of an Old Town, but very neat. Budapest is a cool city.
Bulgaria: Truly a neat country, well deserving of more tourism. The Bulgarians are eager to welcome visitors, and their historic sights, beautiful scenery and relaxed towns are well worth a visit. One caveat: Public transport can be challenging unless you can work your way around reading Cyrllic. Asking around seems to find an English speaker or too (who may hint at needing a little financial help for their assistance).
The Baltics: Tumbleweedz is a better guide than I, as she lives there! However, Lithuania was really cool -- lots of neat churches. And Talinn, Estonia, has gorgeous old town walls that are worth the visit alone.
I say go for it! Lots of these places are VERY amenable to quick, 3-4 day trips which should work into your schedule. Feel free to check out the individual travelogues on each destination on my site, below...
or ask any more questions!