Been a couple of times. Once used Split as a base for a couple of days and went to Krka waterfalls and Trogir, as well as various other places on the way there and back, before heading over to Hvar Island for a couple of days.
Another time flew into Dubrovnik and had a couple of days in the OId Town before getting the boat up the coast to Makarska and staying there for a week. Lots to do activity and sightseeing wise.
Would like to fly into Pula, further North, and have a look round there and Rovinj at some point in the future.
Only Dubrovnik seems to be on high up the tourist map from a UK point of view which surprises me as there are lots of places and islands to visit