3 Week tour of 'Eastern Europe' where would you start?

Am planning a 3 week tour of Eastern European countries next Summer. I have a vague idea of what I want to see and where I want to go. Am planning to drive it in a 1965 VW Camper and am aiming to finish by traveling on the Trans Siberian Express. Any suggestions about places that I may want to stop by? How would you do it and where would you start, finish and go in between?