Planning a trip:

So, one of the things that I can access on my internal internet at work is the State Department's page about travel to every country. So I've been reading some visa requirements and all that for my possible trip in May. Latvia, Lithuania, Estonia, Poland are no problem; however Russia and Belarus could be a bit of an issue (I'm not actually planning on going to all of these countries, these are just sort of all of the possibilities). So I'm trying to figure out what makes sense and what I could conceivably convince someone else to do with me. Romania/Hungary would be cool, as would the Baltic States plus or minus St. Petersburg. Southern Poland and maybe some of the Czech Republic minus Prague? Clearly I'm thinking central/eastern Europe here, but there are just too many options. Help! What can't I miss?