Ok so if you will start in Berlin, you can either go south or east.
Do you know for how many weeks you plan to travel?
I'd say the south route have more to offer:
Berlin Epic city
(Wroclaw?) Epic city
Prague Epic city
(Brno?) ??
Vienna Epic city
Budapest Supposedly nice. (Touristic, so should be able to find cheap flights to most destinations.. Check though)
East is:
Berlin Epic city
Wroclaw Epic city
(Lodz! Kurwa come visit me, but only after 23 of February, got exam! ;D)
Warsaw Blargh, but theres nowhere else to stop.
Vilnius Very beautiful, cheap small city. (I suggest you might want to catch a cheap flight from here to somewhere else. Maybe to Barcelona or smth? Then bus (or expensive supertrains) north, through France and Paris and I guess you might return from Netherlands.)
MINSK The horror! Might be expensive to get out of it, might need Belorussian visa, idk. But you will get to experience the
hardcore eastern europe. Don't flaunt your american-ness!