Question Travel Options from Rome to Slovenia/Croatia

Hello everyone, I'm planning a trip from Rome and considering options to explore Slovenia and Croatia. I’m curious about the best ways to travel between these destinations. Should I go for a bus, train, or maybe even a rental car? I'm looking for scenic routes or any must-see spots along the way. Plus, if anyone has tips on the best time to visit these areas or any hidden gems, I would love to hear them. Thanks in advance!