Dubrovnik to Zagreb

There is frequent, reliable bus service between Dubrovnik and Split, and that's what many people do if the ferry is not an option.

If you rent a car in Dubrovnik, then you can take a ferry to Hvar (or Korcula) if you like. (I'm guessing you will already have seen Mostar?) There is car ferry service from the mainland to Hvar and not just from Split, I believe. (I've not been to Hvar).

If you do a one-way car rental from Split to Zagreb, note that you'll probably pay a higher rate if you rent it for just a few days vs. a longer rental period. This is another reason to rent it in Dubrovnik and keep it all the way to Zagreb: you may wind up paying the same total price for a four day rental as for a two day rental. Driving is easy in Croatia and in April/May the roads outside the towns are not busy, though obviously you pay for fuel, tolls on the A1 expressway, and parking in some cases.

I wasn't a big fan of Split - I spent a few nights there - but yes, some really do like it. Trogir nearby is much smaller and more intimate and charming, but I didn't find it especially memorable, either. I've also stopped in Sibenik (loved it, wish I'd spent more than a few hours), Krka National Park (nice but no Plitvice), and Zadar (spent a night - was nice but not my favorite). I loved Rovinj in Istria but realize that might be more of a detour than you'd prefer to do.

Check out Rome2rio.com for transportation options.

You're right about ferries not running in the off-season. One of the options on Rome2Rio, with the ferry to Korčula only runs in July and August. The RS Adriatic tour stops in Korčula and it was a nice spot to spend a relaxing day and go swimming. It might not be swimming weather in April though. It's still a nice walled Venetian town.

It looks like the bus to Vela Luka runs in the winter. That being said, it's only once a day and it might be nicer to have a car the whole way starting in Dubrovnik.

Also a heads up if you do take the passenger-only ferries: They're smaller, move quite a bit and it might not be easy to see well out the windows. I'd recommend taking seasick pills 30 minutes before departure. Many people in my group got sick on the Passenger ferry from Hvar town to Korčula.

Fortunately, the ferries won't be full at that time of year and you should be able to easily buy tickets once you are in Dubrovnik. Because you don't have to get the tickets ahead, I'd recommend checking it out once you get to Dubrovnik. If no ferries are running or if they are running on the wrong days, just take the bus up to Split. The busses are comfortable, but can feel a little cramped if you are tall. The bus trip from Dubrovnik to Split takes about 5 hours.

Regarding Split, the debate isn't whether you should go or not (you should), but rather how long to stay. If you love museums, culture, ancient history, etc you will want to spend more time there. If you quickly breeze through museums, you'll only need a day or two. When I went through, I was content with about a day and half in Split itself, but I am not the museum type.

Sorry, but some of the info above is incorrect. The ferry between Dubrovnik and Hvar/Split does not take cars and only runs from May so this is not an option for you. From May it runs once a day, arriving in the evening. There are no trains. Buses run regularly to Split.

An alternative to Split and driving up the coast is to fly to Zagreb and spend some time in the north. It won't be beach weather in late April, but should be mild.

The islands will still be very quiet with some restaurants still closed and there is a higher risk of ferries being cancelled if it is too windy. If I were you, I would stick to the mainland at that time of year.