Hi Dave a few things before you go on your vacation. First, in Dubrovnik stay just a 2 days, it will be enough to see everything what you need, than go to Makarska or Baska Voda the most beautiful beaches, very clean see. Take in Makarska ticket for cruise trip and visit Adriatic islands, Hvar, Brac (one day). If you want to enjoy in great night life than you must visit Belgrade (minimum 2 days). You can go to Belgrade from Tivat airport in Montenegro (near Dubrovnik). From Belgrade you can go where you want. My recommendation, take a rent-a-car on Dubrovnik airport, you must book it earlier. Have fun
