Just got back from 10 days in Croatia and loved it! I charted a boat and went from Split to Dubrovnik and back to Split and had a fantastic time. It is a beautiful country with very friendly people! I hope you like "first quality fish" and "mixed grill" because every restaurant serves it

As for Islands Mljet is a very nice place and Hvar is a place not to miss. Another little place Milna is another very nice place with less tourists.
Enjoy the trip