Quite a few years back I did this in a March. I started in Ljubljana, spent a few days in Slovenia for work including a trip up to Maribor where I managed a day of snowboarding. Then off to Croatia via Zagreb and down to Split/Dubrovnik where the weather was beautiful. The coast drive is wonderful and would not be too busy in March. Then into Bosnia for a few days. There are some fantastic mountain roads in Bosnia which follow the rivers. I ended up at the Jahorina ski resort which closed the week before I arrived. I found some drunk Serbian staff who loaned me a snowboard and ran one of the lifts. All in all the whole trip was great and easily done. I did return to Ljubljana to offload the hire car rather than doing a one way.

I have also done Croatia/Montenegro in June and that was still good, even with the crowds. As soon as you move away from the coast it is quiet.