you have a thousand islands to choose from! A lot depends on how much time you have, what you are looking for and the pace you want.
One of our favourite places in Croatia
is not island but an estuary in the mainland: Skradin. We also greatly enjoyed the island of Mljet.
The medieval towns are also lovely: Trogir, Split, Korcula and Hvar. If you have time you should visit Dubrovnik in the south. In the north you have Pula where you have views of a Roman stadium from the marina. Now how cool is that!
A very useful website for high-level planning is JimB Sail | Helping Skippers plan European Cruises