Sounds awesome! I travelled to Croatia and Slovenia (through Italy and Austria/Germany). What are you looking for? Nature, nice medival capitals or the best from everywhere?
I love the nature of Slovenia and the coast of Croatia! I am no expert but love these countries so ask away!
My mix top 9s to be and do are:
1.) Ljubljana: This city is great. Nice food, people and plenty to see! Although I didn‘t find a nice camping right next to the city.
2.) Camp Soča: Next to the river and perfect to hike and bike
3.) Camp Danica: Not as crowded as Bled but nicely situated next to the river. Top!
4.) Pivka Jama: Camping in the forest and a cave, little bit 80ties style facilities but very friendly.
5.) Cycle to Predjama Castle (from the Pivka Jama). Cave builed fortress.
6.) Saksida:
http://www.vinasaksida.com/ great place to stay in the south.
7.) Piran is a nice costal town!
8.) Camp Bunica in Crostia:
http://visitsenj.com/de/touristiches-angebot/campingplatze/camp-bunica-v/
9.) Zagreb is a nice town with history and plenty to see.
Never been to Poland, Slovakia or Hungary so I am also curious what other suggest!