Another option is Dubrovnik-Mostar-Split-Dubrovnik:

The Ciro trail from Dubrovnik to Mostar (Bosnia)
Mostar-Split inland over quiet roads
Islandhopping from Split back to D., eg riding the length of Korçula and Mljet islands, ferries in between.

This is assuming you like visiting old cities/heritage and have no problem doing a little climbing — Croatia is mountainous. Some people don't mind taking the coastal road (EV8) but I strongly advise against, crazy traffic, very unpleasant, downright dangerous. September is not off season anymore.

Islandhopping is even easier in the northern part of the country, between Istria and Split. These islands are not as hilly, but lots of places are very touristy (not off season etc).