We also enjoyed Poland....and Romania & Bulgaria. On the wish list are Lviv & Kiev in Ukraine...(among others)...but the proverbial sands of time are.....
I spent four days in Kiev last month. The city really surprised me. I mainly went to Kiev to go on the Chernobyl tour (which was amazing). I expected the city to a vestige of the old Soviet Union, drab, depressing, etc., and I had a bit of trepidation about safety, especially going by myself. However, it was so far from that. It was beautiful, opulent, safe, and lively. Not at all what I expected. I never expected to want to go back to the city, but now that I've been (by myself), I plan to go back with my wife in the future, and to spend more time there. It's a very large and spread out city without a typical Eastern European Old Town section, so to see all the sights requires a lot of walking. However, Uber is very good and very cheap there, so if you're not into a ton of walking, or can't, it's easy to get around. Of all the places I've visited, English was much less spoken here, so it pays to learn a few key phrases before going.
I've heard Lviv is pretty amazing as well, but I wasn't able to squeeze both cities into such a tight time frame.
If you do go to Kiev, I HIGHLY recommend watching the Netflix documentary "Winter on Fire" (actually, I recommend watching regardless of travel plans). It really made seeing some of the parts of the city seen in the documentary that much more incredible. Plus, the events of the documentary are very much so fresh on the mind of Ukrainians in Kiev, and there are lots of memorials throughout the city to the people who lost their lives in that conflict.
Romania and Bulgaria are both high on our list of places to visit in the future, particularly Romania.