Croatia - 8 Nights

I just did this exact trip backwards. Two nights is plenty for Split. I would give the extra night to one of the islands, either Hvar or Korčula. Both are gorgeous, they just have a different feel. Hvar is more St. Tropez, resort-life and Korčula is more old-world, nothing-has-changed style. You can use either ferry company to get between cities, it just depends on what schedule you want. The time of day you want to travel often determines which company you’ll have to use. The best way to Plitvice is with a bus. RS gives a good explanation of how to do that. I started my trip with two nights in Zagreb and it was plenty to see the sights. There’s not a lot there and it was my least favorite city I visited. Not that you shouldn’t go, but I would personally prioritize the other places.

There are 2 companies operating catamarans between Dubrovnik and Split. One is named Krilo and the other is Jadrolinija. Catamarans going all the way from Dubrovnik to Split stop at both Korcula and Hvar, and both also operate other catamarans that go between the islands and Split. Check all schedules to find the best fit for your itinerary. For Jadrolinija, the catamarans start with the number 9 and you'll need to check both District of Split and District of Dubrovnik to see all the options. The Krilo website is simpler to navigate as they only operate catamarans. I don't believe there's any longer traditional ferry service all the way from Dubrovnik to Split, but there is ferry service for shorter distances. Note that Pullman buses are another option from Dubrovnik to Split, and the views are better than from a catamaran.

If you go straight to Split, you can day trip to Hvar but Korcula really isn't practical. I wouldn't day trip to Plitvice but would instead travel by bus and overnight there on your way to Zagreb (also best for avoiding the most crowded times of day).

I enjoyed my 2-night stay in Zagreb. I found plenty to do, there's a fun pedestrian area in the center of town and the pizza was delicious (right up there with Italy imo). I didn't visit any museums but there are evidently quite a few. You can use the Search box at the top of the page to find prior postings that talk about the museums and other things to do in Zagreb.

My final thought is that 8 nights really isn't enough time to do justice to Dubrovnik, an island, Split, Plitvice and Zagreb, so you may need to edit your destination wish list.

@Bill, great thanks so much that sounds wonderful. Then only 2 nights Split with 1 night, either Hvar or Korčula, I will still do my research. Also though the best way to Plitvice is with a bus. Where can I find more details on that? Can you do this as day trip? I don't have the RS guidebook. Personally also not interested in Zagreb, just thought its convenient to sleep there when doing Plitvice and fly out.

Plitvice gets lasted by day-trippers to the point that the visitor experience is seriously degraded. The park was mobbed when I day-tripped from Zagreb, and I think it takes longer to get there from Split.

Arrive to the lakes in the evening and get an early start next morning to avoid the crowd.
http://www.ak-split.hr/en/ Is your most accurate guide.

We just returned from Croatia/Slovenia last night. We found 2 nights in Split was enough. Whatever ferry you choose, be sure to book 2 days in advance. If you want to go to Hvar or Korcula, the ferries stop there on the way to Split so you could break up the journey there then carry on. I never would have considered going to Plitvice if I had to rely on public transportation; we rented a car in Split, then toured the park from about 4-7:30. Don't go mid-day or it will be packed. We really liked Zagreb and it wasn't packed with tourists. We spent one night there, which is enough to do the Rick Steves walk with stops at the Museums of Naive Art and Broken relationships, both excellent.

Have fun!