3 weeks in Croatia/Slovenia

My wife and I are planning to do 3 weeks( + or -) covering Northern Croatia and Slovenia. Arriving in Zagreb, our planned route includes Ljubljana, Bled and Piran (and possibly) Maribor in Slovenia as well as Rovinj, Pula, and Zadar in Croatia.
Is this doable using public transport ( buses and trains) or would we require a car for some of the route since we also wish to hit Skocjan Caves and Predjama Castle in Slovenia as well as Plitvice Lakes in Croatia. Any links to affordable local guides and tourist agencies would be appreciated. we have used G Adventures and Insight on previous Balkan tours but are finding their itineraries a little too rushed ( one night stays do not allow one to see enough of a destination) BY way of a backgrounder, we are in our mid 60's but are fairly active and travel light.

It's perfectly possible to do all by public transport (with some backtracking, however), but a car would enable you to drive along more scenic and shorter itineraries, for example
Maribor - Bled via Drau Valley, direct drive to Predjama museum and caves as well as to Skokjan Caves, more scenic and faster itinerary from Piran to Porec and Rovinj, Istria - Plitvice without several bus changes, etc.

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