Critique my Agenda Please

My first thought is the car. Renting in one country and dropping off in another can be very expensive, if even doable between these two countries.
I would leave the car in Ljubljana and either train or bus to Zagreb for one night (take away from Korčula). Then pick up a car in Zagreb leaving it in Split.
Maybe forego the car in Slovenia and look for a day trip to Postonja and Predjama. Would save some aggravation and time.
When in Rovinj it is difficult to park so research this ahead of time. Maybe on booking.com research a place with parking. You could also look to stay in Poreč, close by with less tourists.
While in Istria, look to visit Pula, Groznjan or some other hilltop town. If you start out early, you can easily visit 2 in one day. We visited both Groznjan and Oprtalj in one day - you can drive up to the top, parking ample (small fee in Groznjan, free in Oprtalj. The latter has a fabulous restaurant, Konoba Oprtalj, with glorious views).
It looks good if you are agile enough for what I call fast travel. We tend to stay at least 3 nights in a place.
By the time you get to Dubrovnik you might be slowing down. If you miss the day trip to Cavtat it wouldn’t be too big a deal. It’s a lovely coastal town, but so is every place along the coast. Spending two days in Dubrovnik wouldn’t be a hardship.

Personally I liked Zagreb better than Ljubljana.
Otherwise, as you asked, another thought is that it is one day each place, too much driving. It's a whirlwind.

OK, some revisions and would really appreciate a critique.

Day 1: Arrive Ljubiana
2: Rent car, Lake Bled
3: Caves and castle. Return car.
4: Bus to Rovijn, stay there. Rent car.
5: Istria
6: Istria
7: drive to Plitvice
8: Hike lakes all day, then drive to Split
9: Return car, explore Split
10: Split
11: Boat to Hvar
12: Boat to Korchula
13: Korchula
14: Korchula
15: Boat to Dubrovnik
16: Dubrovnik
17: Bus to Cavtat day trip - should we stay
18: Fly out of Dubrovnik

Any and all suggestions appreciated.

I agree with Barbara; there are buses that go to Postojna/Predjama from the bus station
in downtown Ljubljana so you wouldn't need a car for that. If you want to go to Skocjan,
that may require a car. Similarly for Bled, I think you can take a bus.

You probably want a car if you are thinking of doing the Julian Alps, though.

Agree that Cavtat is optional for sightseeing.

But most of all, I agree that this itinerary seems to be constructed to try and see everything
in the time you have available, and the likely result is that you will spend a lot of time getting
from place to place and less time seeing the places.

Given how you've allocated the time, you might consider dropping Slovenia entirely, flying
into Zagreb, and go from there to Plitvice, then Istria/Rovinj. A little less driving and you
start/end in Croatia, so less car rental hassles.

Day 7: you might arrive in time to do start of Plitvice this day
Day 8: see if you can arrive in Split in time to return the car late that day.

There’s nothing wrong with this plan. It IS a fast-paced trip, but doable. You ARE rushing through Slovenia, so mentally allow yourself the freedom to change things up if you feel like it: linger longer, skip a cave or a castle - if you decide to. And as Barbara said, give yourself permission to skip Cavtat if you just want to sit and enjoy a final day in Croatia with no agenda.

I say all this with the assumption you will be there during longer days.

As others have stated, there’s still a lot on the agenda. This is really the crux of the “problem” with this region, as I’m sure you’re feeling the transportation gets complicated fast.

Some thoughts:
You really hit the ground running in Ljubljana, and I think you could use an extra day here. It also looks like you’re in Dubrovnik for two nights? Three would be ideal. I think you can steal a couple nights to broaden these bookends. There’s zero need to stop in Hvar for a night if you’re staying in Korcula too.

Istria… is an outlier in most itineraries. It takes some extra effort to reach, and honestly most of what I read about Istria in general is that it’s kind of middling compared to the rest of your agenda. Without this, you’re not particularly obligated to rent a car (though it helps with Plitvice.) And you can redistribute some of those “coast days” to the other coast destinations. It’s a drastic change to your itinerary, but axing this area altogether really loosens things up in a good way.

There’s numerous day trip operators that run a full day Caves and Castles tour out of Ljubljana. This would absolutely be the way to go imo, especially with the itinerary as tight as it is.

OK thanks everyone - so the feedback I am hearing is that I am trying to jam too much in. I could either make this a longer trip or cut stuff out. Let's try the latter first.

Day 1: Arrive Slovenia
Day 2: Explore Ljubljana on foot
Day 3: Bus to cave and castle
Day 4: Rent car, drive to Lake Bled in morning, afternoon back in Ljubiana
Day 5: Train to Zagreb - explore Zagreb afternoon
Day 6: Zagreb all day
Day 7: Rent car, in Zagreb drive towards Plitvice and stay in Rastoke
Day 8: Hike lakes (8 hour hike?). Stay the night in Rastoke.
Day 9: Drive to Split, return car
Day 10: Split
Day 11: Split - sleep in Split
Day 12: Boat to Korcula
Day 13: Korcula
Day 14: Korcula - sleep in Korcula
Day 15: Boat to Dubrovnik
Day 15: Dubrovnik - walk walls in morning
Day 16: Day trip to Cavtat
Day 17: Dubrovnik
Day 18: Fly home

Is this one better? I can add a night here or there. Any stop seem too short or too long?

OP, at some point you will just want to go with whatever you have; it's your trip,
not the forum's trip. People are happy to comment and advise but you probably
should not expect everyone to give their stamp of approval.

My only other comment for now is that buses are worth considering as a mode
of xport. You should be able to do a day bus trip to Bled, and also bus is a easy option
for going from Ljubljana to Zagreb.

Agree with shoeflier. This is doable and not so rushed. If you have another day and want to add it, add it to Slovenia.

Rather than seeing Cavtat on a day trip, I’d stay in Dubrovnik on day 16 and move to a Cavtat hotel on day 17, finishing up your trip there. It’s a lovely, peaceful little village. You can spend your last evening sitting by the harbor at sunset with a glass of wine, watching the yachts dock. Then in the morning, it’s just a few minutes to the airport.

You have a wonderful itinerary planned. Have an outstanding time.

I like your latest a lot more, Gary. In Slovenia, there’s regular bus service between Bled and Ljubljana. I’m just saying a rental for either day isn’t necessary, BUT if it’s all the same to you I would certainly say a car is a nice convenience to have.

Cavtat is a nice little town, but it’s not essential. You can “see it” in a couple hours. They have some nice quiet beaches and good restaurants. Play it by ear. If Dubrovnik is crazy and/or not to your liking then sure.