Just to share my experience:
Zagreb: quite small city center, perfect for strolling or shopping, and for cafe-loving persons. I believe every croat has a shoe-fetish - extraordinary many shoe-stores. Try the restaurant Kaptolska Klet (spelling?) for "country-side eating" and entertainment. For a classy meal, visit the restaurant at Regent Hotel Esplanade. Expensive but great. We stayed at Arcotel Allegra - modern and good service.
Opatija: very touristy. Relatively expensive, both for shopping and hotel-wise. Very warm water for bathing. High comfort.
Plitvice national park: excellent nature, even for a norwegian like myself.. A MUST-SEE. Stay at small B&Bs just outside the national park (f.ex. Marco tourist center). Enter before noon (warm in afternoon, and plenty of old-people coming in on busses). We took the 3-4 hour route. Should have taken the 5-6 hours (slow hiking).
Split: good for short visits and shopping. Not the best air (many ferries and cars polluting). Happenings in summertime. Hotel Split was perfect, 20 minute walk from city center.
Bol: touristy, but good. Plenty of nice restaurants, "best beach in Croatia" stills means pebbles/rocks. We stayed in private accomodation. Very good value. Check out adriatica.net. Nothing to buy. Restaurants pretty much "all the same", which means good.
Jablanac: probably unknown for most. Very small place. Hotel Ablana, half board. Great food, 3 course dinner. Perfect place for a few days relaxing. Sunbath on concrete beaches. Swim in the bluest water pretty much by alone. Let the ferries to Rab pass you as you splash around. Picturesce.
Ferry to RAB: just an advice: never take it on saturdays during summer. 4 hours waiting time. The day before or day after, 20 minutes waiting time.