Advice for Croatia

Option 1: Istria + Kvarner Gulf ; maybe drive back up to Piran, Slovenia for last night, drop the original Slovenia car up there (to avoid a steep one-way drop fee for dropping the car in a different country). There is a catamaran from Piran too in summer I think; otherwise, just take the bus or GoOpti back to Venice. Piran is a lovely town itself.

Option 2: find a direct flight from Split to Venice (They exist, maybe not every day.) Then you can drive from Plitvice to Split, catamaran to Hvar, back to Split and fly to Venice. Either pay the one-way drop fee or rent two cars, one in Slovenia one in Croatia, train between Ljubljana and Zagreb.

There's good rail and bus service between Ljubljana and Zagreb, so I think you could drop the Slovene car in Ljubljana, take public transportation to Zagreb, then pick up a new car when you've finished your Zagreb sightseeing. If the cross-border drop charge is as high as I suspect it would be, the minor inconvenience would be worth it.

The best course of action regarding the car will depend on how the catamarans run. Early June is not high season yet so you may find they do not run every day (I haven’t checked, next year’s timetables aren’t online yet). If you’ll be taking the catamaran from Pula or Rovinj, it’s best to drop the car in Ljubljana and take the bus or train to Zagreb. Rent another car in Zagreb and drop it off in Pula or Rovinj. Alternatively, if taking the catamaran from Piran, stick with the Slovenian rental and drop it off at the end of your trip in Piran.

As far as the itinerary goes, I think the Istria and Kvarner combination is the right choice. I like Mali Losinj a lot, I always enjoy visiting. The town is a nice size with a lovely town centre and there are other places to explore on Losinj and Cres islands. As alternatives I would also look into Rab and Krk, not Hvar. You could then end your trip with a couple of nights in Istria.

There are 5-6 great Croatian towns S of Plitvice Lakes and not too out of the way: Split (a little south), Trogir, Zadar, Sibenik, Rovinj. 2 nights in Zadar might be fun. I don't see how the islands would fit into such a compressed time frame. I would go to PL, and then S to the coast, maybe 2 nights in Trogir, 1 in Zadar. The Croatian coastal towns are all quite unique, but there is a strong pattern to them - all made of limestone block, all with nice little cafes, all with friendly Croatian hosts. But if you go to Trogir or Zadar, you get the flavor. Dubrovnik is the best of them, although Kotor is also pretty cool. But those are quite south.

Rovinj is WNW of Plitvice Lakes National Park, in the Istrian Peninsula. I like that area a lot (as do others who've traveled there), but having spent a good bit of time in inland locations, I think you might enjoy a bit of time on a Croatian island.

Reading this post with great interest as your plans are very similar to mine -- except I'm going first 2 weeks September and probably won't rent car. I had originally thought I'd make it to Dubrovnik but like you, thinking it just takes too much time to get to/from. I agree that it sounds fun to take a catamaran. Hope you post a trip report when you return.