Croatia is a great place and during August, the weather should be wonderful for a sunny holiday. A drive holiday sounds great but 6 days might be too short to experience it all. You could look at a Dubrovnik to Split drive but if feeling adventurous, try driving up to Zagreb.
Start off at Dubrovnik. If you’re a Game of Thrones fan, the old city is where you want to be. From walking the wall to discovering hidden locations, Dubrovnik can be extremely busy but also, friendly. You won’t need a car there.
However, once you head out to Split, you can pick your desired vehicle (sixtcom, rentalcars.com). The road trip to Split takes about 4 hours if you divert and head to Trsteno Arboretum on the way. Again, beautiful place but a GoT fan would definitely want to visit the gardens at King’s Landing.
Split is interesting with lots of things on offer. Beaches, Diocletian’s Palace, Marjar Hill, great restaurants, you decide what you want to do.
Base yourself in Split and then take a ferry to Hvar, about two hours away. You could decide to stay in Hvar but my suggestion would be to keep it as a day trip.
Then there’s also the drive to Plitvice Lakes, about three hours away from Split. It’s a national park worth visiting because of the extreme natural beauty it offers visitors.
While driving in Croatia, remember that it is right-hand drive and the motorways are good. However, in some areas, the highways are replaced by two-way country roads. So be careful.