Gah, definitely go to Split on the Adriatic coast! I want to but I don’t think I’ll be able to practically fit it into my upcoming trip. The whole town is basically situated inside an ancient Roman Palace!

I never been to Croatia so i cant help you that one, but Poland is a must see. I know when people go to Poland, they only go as far as Krakow. I would say go to north as well. My fav cities were Gdansk, Torun, Wrocla, Warsaw and many more beautiful small towns on the way. On top of it, its very cheap country compare to western countries so thats always a plus.

Zagreb isn’t usually at the top of anyone’s list, most people go to Croatia for beaches and coastal cities.  Although very touristy, Dubrovnik looks really amazing.  If you’re going that far south, Montenegro is beautiful also. 
 
 
I’ve been to Poland twice now and will not hesitate to go again, trying to work a couple cities into my fall itinerary actually.  City for city, Poland has as much culture and as many historic sights as anywhere in western europe.  They have good, hearty, traditional foods and tasty beer and spirits.  A lot of bars and restruants are underground, sounds a little tacky, but the places are actually upscale and make for really cool atmosphere. 
Many of the old town squares, known as a rynek, contain beautiful clock towers and lots of renaissance style architecture.  Zamosc in eastern Poland is considered to be one of the most beautiful renaissance cities in all of europe.  A lot of ryneks are UNESCO world heritage sites, and the one in Warsaw is one of the only completely rebuilt structures to become a world heritage site. 
As mentioned before, Krakow is the extent of most travelers’ visit to Poland.  Its an amazing city and has top notch day trips, like Auschwitz concentration camp and Wieliczka salt mines.  The downside of Krakow is that its loaded with tourists, fortunately there are plenty of worthwhile cities throughout the country that few tourists ever venture to. 

Agree.
Dubrovnik is worth it, despite the crowds.
We ferried to the island of Hvar (Dalmatian coast) and enjoyed it quite a bit. 
Have heard good things about Korcula as well.
Coastal Montenegro is beautiful.