I rented a car from Ljubljana and returned it the same day. I found the roads to be excellent, well signed, not very busy, and easy to navigate. Traffic was surprisingly light and getting to and from the sites we visited was stress free and quite easy. Slovenia is not as populated as most European countries and driving in and out of Ljubljana and around the countryside was very easy. Very clean and well maintained streets, highways, and beautiful countryside. Driving there was no more difficult than driving a medium sized US city…. So car rental may be worth taking a look at.
Otherwise - tours by locals offers a lot of options and most guides are willing to customize itineraries. Just google tours by local in Slovenia for the site.