Croatia, Bosnia, Montenegro

You have a sensible timescale for your itinerary. I would encourage you to spend the first night in Dubrovnik, assuming you are off a long transatlantic flight. The last thing you will want if tired is a long drive, allowing for border crossings.

Dubrovnik to Hvar is a catamaran, not a ferry and should be booked online in advance at krilo.hr. I prefer Stari Grad and Jelsa to Hvar town, which will still be in full party mode.

The Korcula catamaran daily service doesn’t give you much time there, but there are local dayboats that go to Brac from Vrboska, departing at breakfast time and returning at 5pm.

Trogir can be covered from Split.

Istria deserves a week, which you could manage in your timeframe.

I would reverse the order of the trip, as the weather will be better in the north in August, rather than September.

I like Jennifer's idea of starting in the north. Zagreb's a good gateway for flights, and there's lots to see there, so there will be no need to get into a car and start driving right away. I particularly would not want to drive from Dubrovnik into Montenegro in a sleep-deprived/jetlagged state. The scenery along the Bay of Kotor is magnificent and the road requires attention.

There's enough to see in Montenegro that I'd want more than "a few nights" there, but I suspect that's all the time you'll have. Time permitting, I'd like to visit Herceg-Novi, Ulcinj and Cetinje in addition to Kotor. I didn't like Budva as well as Kotor, but I was lucky to have some time in Kotor before the day the mega-ship showed up. If a big ship is in port at Kotor, Budva would be more pleasant.

Thank you both for your replies. It does make sense to spend one night in Dubrovnik before starting out. I was thinking of starting with Montenegro and Bosnia, then three nights in Dubrovnik and heading north so to end up in Istria to relax a bit at the end of the trip. Will the weather be more fall like then in Istria? We aren’t ones to sit on a beach for long, prefer more seeing the sites and enjoying the culture, so fall weather would be ok...it would be late September. And where is the best base in Istria? Also, any thoughts on taking the ferry from Rovinj to Venice? Many thanks!

No one can give you guarantees about when the weather will change. The issue isn't really the temperature, which should remain very pleasant as long as it's dry; it's the possibility of rather a lot of rain. During transitional seasons, the standard advise is to schedule the more northerly spots when it's likelier to be warm, but it may well turn out not to matter while you're there. I can only say that during my 2015 trip I began seeing rain in Bulgaria in late September, got quite wet in Montenegro on October 5, and got drenched on a chilly day in Zagreb on October 11. Those were not the only days of rain during that period.

All good. You will see best of it in no rush.