The drive from Split to Plitvice is mostly inland if you take the fast A1 highway (look on a map) - or scenic if you take the very slow coastal highway. Most people take the A1 I think, and you'd probably get your fill of coastal scenery between Dubrovnik and Split anyway. There are some towns you can stop at between Split and Plitvice (like Sibenik and Zadar, also Krka National Park near Sibenik). Depends how much time you have to spend.
From Plitvice, the easiest ways to get to Ljubljana by public transit are probably from Rijeka or Zagreb (direct train service to Ljubljana and good rental car coverage for dropping a car with various companies). Zagreb has a few more trains a day (plus direct buses too) but could be more of a hassle because Zagreb is bigger than Rijeka. And Zagreb is slightly closer to Plitvice.
You could also visit Istria - stop in a town like Rovinj (loved it), drop the car there, take a direct bus to Ljubljana. Or you could combine Rovinj with dropping the car in Rijeka - but that's some extra detour (look on a map). I in went the opposite way: Ljubljana to Rijeka by train, rent a car in Rjeka, drove to Rovinj for a few nights, back through Rijeka and south to Zadar etc. Whether you want to spend a few days in Istria depends how much time you have. I love Slovenia too and wouldn't recommend cutting your time short there, either. (I might also rent a second car there even if just for a day from Ljubljana to do scenic drives between there and Bled).
