Croatian Itinerary

Hi. I would recommend seeing Split, Hvar, Korcula, Plitvice for the National Park, the capital city Zagreb, Krka for the National Park, Pula for the ancient Roman architecture and Zadar on the northern Dalmatian coast. Croatia is a beautiful country and has a lot to offer. Have fun!

It will be easiest to rent a car if you can. Driving is easy in Croatia. It's a simple drive mostly up a modern highway (toll road) from Split to Zagreb, with a detour off of it to get to Plitvice.

You could take a catamaran out to one of the islands like Hvar from Split if you want to start there, then come back to Split by catamaran, rent a car, and start driving north. Not sure if 12 days means 11 nights - 10 nights in country + 1 night overnight on the trans-Atlantic flight? If you have ten nights, maybe I'd do:

Split (2 nights)
Hvar (2 nights)
Sibenik (1 night) - base for Krka.
Zadar (1 night)
Plitvice (1 night)
Zagreb (2 nights)

That is nine nights - you would have one more to add somewhere else (Split or Zagreb or Hvar if you want to start the trip with a few days just to relax). You could detour to Istria (Pula, Rovinj, hill towns) between Zadar and Plitvice if you really have 11 nights in country. Or chop one night off of Zagreb at the end - but I think you would want two nights in Istria at least to make it worthwhile. People are split on Zagreb - I wasn't a fan myself but some people do like it. It is definitely worth seeing. If you could depart Plitvice by mid-afternoon you'd still have a few hours the final evening to explore Zagreb at the end. You'd want at least half a day to explore the gist of Zagreb.

Krka and Plitvice are both beautiful sights, would highly recommend visiting them if possible. Also, as many major islands as you can - each of them gives off a particular vibe!

I would also say a strong yes to spending more than one day in Zagreb. It's not coastal, yes, but it is beautiful in its own way, not to mention the history and culture of its own.

I think the bus (+ ferry for islands, of course) would be the best bet. Trains are usually slow and often not that cheap, and I don't know if rent a car would pay off for such distances (you'd have to check it to be sure).

Landing in Zagreb
Day trip in Zagreb to see the main square and upper town. Then going to visit Plitvice Lakes. If you like nature, you can book a few days accommodation at Plitvice. South islands Hvar, Korcula are paradise on the earth. The bus is super fine to use. On the way from Plitvice to the south, you can visit all the places you have on the list.

Which month?
Did you just visit Dubrovnik the last time?

Sample itinerary flying into Zagreb and out of Split (or vice versa)
Zagreb 1-2 nights
Plitvice 1-2 nights
Krka area 1-2 nights
Vis (via Split) 3 nights
Split 1-2 nights

(Note I am not recommending a series of one-night stays, these are just estimates for the minimum you'd want and you should adjust according to your interests.) You could do all of that with bus/boat, or you could rent a car for portions. You could stay in Split longest and do day trips, but I really recommend at least three nights on Vis to give you two full days (and renting a scooter while there)--it takes longer to get there so make the most of it.