In Slovenia and Croatia you have a lot of choices where to go. This two countries also offer you some greenlining around. If you will stick only on the highways, It will become boring in a few days.
In Slovenia I recommend that you go to the Soča Valley and then on Mangart road (if it will not be closed already - the highest point is above 2000m high.), Vršič mountain pass, and visiting valley Vrata under mount Triglav. If you will be in the mountain area you can also visit Lake Bled and Lake Bohinj (about 25km from Vrata valley.)
From the tourist destinations I will recommend Postojna cave and Predjama castle or even Škocjan Caves near Divača. Also a lot of opportunities for greenlining around.
If you will go in Croatia and you will be lucky with the weather, I recommend you to visit Velebit mountains. (Go to Senj or Gospić, and then to Štirovača and passing Ravni Dabar to Baške Oštarije and go further south to Paklenica and Stari grad.
Other option is to go to Plitvice lake and visiting old Yugoslav army base Željava on the border with Bosnia.
If you will need some info, you can contact me on pm here or on email.