We spent 9 days exploring Slovenia and Croatia and I wish we had more. We did rent a car for the whole time and Croatia has wonderful expressways. And we did pay a large fee for picking up in Slovenia and dropping in Croatia. It was unfortunate, but the trade off was less headaches, save time and more flexabilty. Slovenian roads are hairpin turns and constant switchbacks. Take dramamine if you get car sick!
We stayed in Bled, Slovenia. Beautiful, relaxing but not a lot to do in town other than go over to the island with the church. We took a day trip to the Julian Alps (fabulous! except the car sick thing). There was a WWII museum that was very impactful - highly recommend stopping at it (small town that I can look up if interested).
Then we drove to Plitvice National Park in Croatia. Stunning beauty. 100's of natural waterfalls. Green and lush.
Then we drove to Split - port city with interesting history.
Then we drove to Dubrovnik - we took a chilly dip in the Adriadic here (it was late Oct). Best memory is the seafood restaurant we ate at in the center of town. Oh how I wish I could go back just for that!
Then we flew home. If I had more days - I would have really wanted to get over to the Istrian Pennisula. I just didn't have the time.