I'll be doing a similar route over 3 weeks from around 21st August to meet a friend in Ljubljana. Just pretty much going through Germany and Austria and seeing what there is to see along the way.
In Slovenia I'd just suggest taking a day to look around Lake Bohinj and the Vrsic pass from Kranjska Gora to Tolmin, a gorgeous part of the world with loads of little mountain huts to grab some food and a
. Also just stay off the motorways if you can, the roads closer to the Croatian border run through some gorgeous sections that get increasingly twisty and hilly.
For Croatia, I've only been there once, seems nice if a bit more hot and dusty than Slovenia. Personally, Dubrovnik was a huge dissapointment, all t-shirt shops, top dollar cafes and unbelievably packed in with as many tourists from the cruise ships as you could possibly fit. Maybe it was just a bad day!?
Brac and Hvar islands are stunning - boat from Makarska, as were the Krka waterfalls by boat from Skradin.
When you get an idea when and where you're going, give me a shout