We spent two weeks in Croatia last fall, and felt like we barely scratched the surface. Sorting out what you're interests are will help you a lot in figuring out your itinerary. We really liked Split for the museums, sights and restaurants. Plitvice is really beautiful and unique. Even if you're not a nature-lover, it's worth the effort to get there. Make sure you book in advance, as there are no ticket sales at the entrances. You can comfortably do it in a full day. We stayed overnight at one of the park hotels, Hotel Jerezo. It was a hoot. Straight out of the Tito era property: big rooms, 60's decor, easy parking and free second day access to the park, which is helpful if you arrive after lunch from wherever you're coming.
Of the big cities in Croatia, I enjoyed Split the most (although Zagreb was a close second). Split has interesting museums (the ethnographic was my favorite), a large Mestrovic museum/gallery, Diocletian's Palace, and some great restaurants. I much preferred it to Dubrovnik, which was a zoo. Too many people for small spaces was the biggest complaint we had. The wall is interesting and there are pretty views. Add a couple of churches, a couple of war museums/exhibits (the best being on Mr Srd near the lookout), and that's pretty much it. Not worth the effort IMHO.
I loved Rovinj and some of the hill towns are really cute (although I thought Motovun was overrated, both when we visited 5 years ago and now--but many people here will disagree). Driving in Croatia is really easy, but we spent more time in the car than we had planned for. Croatia is bigger than it looks.
It really depends on your interests. Museums and art -- Split and Zagreb. Beaches? South of Split and/or islands. Nature, definitely Plitvice. Hill towns with Italian influence and quaintness factor is Istria. Depends on your time, too. With a short visit, I'd see less and enjoy it more. Round-tripping to/from Split makes the most sense to me.