I could hardly add anything to what John Rule said about Slovenia. He is well informed. I called Kompas Hertz (the car renting company here in town) and they confirmed that some EU member countries (mostly Italy) might not allow their rented cars travel into the EU non-member countries. It has to do with the fact that relative insurance companies might refuse to cover insurance outside the EU member countries. Usually this does not happen with Austria or Switzerland but it does with Italy. Anyway, the Hertz company here in town resolves this poblem by sending the driver with the car to Ronchi (Triest) airport to deliver the car to customer. The driver brings customer to Slovenia where he can continue the ride on his own. Same procedure applies on the way back. Let me remind you it's only a short ride (a little more than half hour) from here to Ronchi airport.
About the caves of Postojna and Skocjan. Visitors don't have to climb or do anything special, no special skills are needed other than taking a walk or a ride in a small train within the cave. In my opinion both caves are worth seeing.