Where are the best places to visit in Eastern Europe?

Thinking about going there with my wife in August, We've been on two vacations to Europe and loved them, but this would be our first one to eastern Europe.

I love Tallin, Estonia (old town, townhouse, windy cobbled streets with many interesting shops, Russian orthodox churches) and like Vilnius, Lithuania (architecture and nice people) but didn't enjoy Riga, Latvia, so much. Maybe it was due to the daily rain when I was there. But as I was there in autumn (November), the circus was in town, and it reminded me of the Barnum and Bailey show-type of circus as opposed to what we have nowadays, e.g. Cirque de Soliel.

Poland is definitely worth going to; Warsaw - old town and interesting architecture, Krakow - market in old town, cobbled streets, salt cathedral/spa/ex-mine, Austwich and Berkanau - history.

Yes, Warsaw is nice, Krakow I found not that interesting. If you are visiting the Czeck Republik, Prague of course.

Romania

Prague, i'm in love with this city. It is so beautiful, theres lots to do and it was really really good value. I wasn't crazy about the native food, lots of meat (not so great for a vegetarian)and pickled things but there are lots of italian & french restraunts etc.