Slovenia/Croatia Itinerary Critique

Overall, looks pretty reasonable.

Note that in April, expect the weather to do anything - we went in May, and the roads in the Julian Alps got a major snowfall the day before our planned driving trip up there from Bled. They cleared the roads fairly quickly, but be ready for surprises. Also be very careful about ferry (and even bus) schedules - many routes have reduced service (or no service at all) outside of "high season" (= July+August).

I might take issue with some minor points, eg:

Day 8-9: best plan for Plitvice in my opinion is to arrive there mid-afternoon, check in to your hotel in the park (not the nicest place you'ce ever slept, but all about location location location, and beating the crowds). Late afternoon, go out and enjoy the park for a few hours as the crowds melt away. Sleep in the park hotel, get up early, spend a few more hours enjoying the park before the crowds show up, then depart late morning for your next destination.

Day 10: We didn't care much for Hvar (town). We loved Korcula though, it was perhaps our favorite place in Croatia (tough competition). Consider swapping Korcula in place of Hvar (unless you want to dance until dawn).

Day 12 or 13: For one of you day trips from Dubrovnik, rent a car locally and do Kotor.

For both Dubrovnik and also Kotor, check cruise ship schedules carefully and try to avoid either city when they are at their worst.

Overall, I think your plan is reasonable. Have fun!

Day 5: if you want to do most of your driving down towards the Skocjan Caves on Day 5, you could stay in Trieste, Italy, which is just 25km from the caves. Trieste is even closer (13km) to Lipica, home of the Stud Farm that breeds Lippizan horses.

http://www.lipica.org/en/stable/stud-farm

From Trieste, you can easily visit both the Lipica Stud Farm and the Skocjan Caves on Day 6 before returning to Ljubljana. Check the Lipica website for the days of their tours and (not daily) training sessions.

Edited to Add: driving from Lipica to the Skocjan caves, there is a lovely little restaurant, Restavracija Okusi Krasa, that is surprising in quality for its out of the way location. The prices are not inexpensive, but reasonable for the quality. Not open daily, check website: https://www.okusikrasa.si/

Looks like a great trip!

Day 5: if you'll be driving on Slovenian motorways, be sure to pick up your Class 2A (auto) vignette for 15 Euros (2019.)

https://www.slovenia.info/en/plan-your-trip/getting-to-and-around-slovenia/to-slovenia-by-car

Seams like you have Slovenia part of your trip covered in the answers above so I will focus on Croatia. First, I would rather go by bus from Ljubljana to Zagreb. Evening and one full day will be enough for sightseeing of Zagreb. Stop at Plitvice Lakes is definitely a must do on your way south. Since you will have a rental car I suggest you do several short stops along the way like stop at Slunj. You can see more here https://mappingcroatia.com/croatia-itinerary/plitvice-lakes-national-park-day-trip-from-zagreb/
Boat ride to Dubrovnik with stop at the island is a great choice. This way you will avoid border crossing and get the best out of your journey.
Someone mentioned you should go to Korcula instead of Hvar - while I personally think Korcula really is a better choice it is a smaller island so in April you might find a lot of restaurants etc still closed.
On the other hand, April is a perfect time to visit Dubrovnik. It is still low season so you shouldn't encounter huge crowds. As a day trip from Dubrovnik I would choose Kotor in Montenegro or beautiful Peljesac peninsula if you are a wine person. Trip to Montenegro is a full day adventure and you will probably be exhausted so trip to Peljesac the other day (which is shorter, more of a half day trip) is the best option. For second trip you should also consider visiting southernmost region of Croatia - Konavle and city of Cavtat. Hope I helped:)

Overall I think your itinerary is great. Happy to see you are spending time in Ljubljana, our newest favorite city. I agree with previous contributor to arrive at Lake Plitvice in the afternoon, do some exploring, stay the night at a park hotel. In am get an early start. Our tour group started at the "top" and worked our way down. This avoided the usual crowds starting from the lower end of the lake. Hvar island was our least favorite. We found Korcula was unpretentious, less crowded, fine cuisine and relaxing. We spent two nights there. Visiting Dubrovnik was another favorite. That said we avoided the crowds by touring old town in the early morning. We felt two nights were perfect. If time permitted we would have extended our stay in Montenegro.
Have a great journey!

** Edit- We were at Lake Plitvice 6 months ago. Yes, by mid morning the crowds came in!

The others have given you solid advice. Listen in particular to the advice about Plitvice. I saw it before the throngs discovered it and I saw it during day-tripping hours in 2015 (I'm sure it's worse now). There is simply no comparison.

Another good side-trip from Dubrovnik is to Mostar in Bosnia-Hercegovina, but Sarajevo is also extremely worthwhile, so I'd hold off on both of those until your next trip when you can spend some additional time in that area. Montenegro also has a lot of other attractive destinations. Plus there's the Istrian Peninsula in Croatia.

If you opt to go down to Montenegro one day, you'll find the ride around the Bay of Kotor really stunning. I don't think you'll have a great deal of extra time, but if you do, I enjoyed a couple of hours wandering around Herceg-Novi, which is right on your way. Others have enjoyed Perast, which is also on the way. I visited H-N while staying in Kotor and am not sure it would work if you needed to return to Dubrovnik. There are some Dubrovnik-Kotor public buses in the summer, but I don't know what the situation will be like in April. You might be limited to bus tours.

I don't remember the exact dates. I think both my earlier trips were in the 1980s. The earlier trips were not in July or August--either June or September. I remember absolutely no crowding, whereas on Friday, August 28, 2015 there was a solid mass of people on the walkways. If one person had stumbled that day, probably half a dozen people would have ended up in the lake. On all my visits I day-tripped in from Zagreb or Split. I would not do that again.

Simple and nice Itinerary. I will suggest sleep one night at Plitvice and visiting a few south Croatian islands.

I'm sorry it has taken me so long to reply but thank you to everyone for your tips and advice! They've been great!

Our only major change has been to unfortunately cut out Hvar because we would be there on a Sunday and there is no boat leaving from the island to our next destination, Dubrovnik, that afternoon or evening. Now we just have to come back another time to do the islands! Our day trips from Dubrovnik that we have planned are to the Bay of Kotor in Montenegro and to Mostar in Bosnia-Herzegovina.

Thank you again for all your help!