Croatia Itinerary Help approximately 12 nights for 2 adults with no children in mid 50s

Personally, I think 4 nights is too many in Dubrovnik. The wall, a couple museums and churches, Mt Srd and you're done. Two days, three max, would do it I think. Split is definitely worth the time. The Diocletian Palace and the Mestrovic Gallery are both must-sees in my book. Perhaps others could give you some insights on Hvar in September. That's not high season, but probably would still be ok weather-wise. Since you've got 12 days, you might want to consider Plitvice Lakes National Park and Zagreb. The lakes are beautiful and Zagreb, Croatia's largest city and capital, is rarely visited, but worth a couple days for the museums (especially the Museum of Naive Art) and atmosphere. You'll get away from the crush of tourists in Dubrovnik and get a completely different view of the country. Both are a bit of an outlier when coming from Dubrovnik/Split, but worth the effort to get there.

Make sure you add in Plitvice! We have visited over 100 countries and these waterfalls rank in the top 10 experiences we have discovered!

Stay around the Park the night before so you can beat the day trippers the next morning.

It is truly magical.

We had a rental car and rode the car ferry from Split to Hvar, then drove off the island (rode the car ferry from Sucaraj to Drvenik) and drove down into Dubrovnik.

But friends visited Korcula (day ferry from Hvar) and noted that it was a beautiful island to visit also.

You do not need to prebook ferry tickets in September. However, we always do for a few reasons - it’s one more item off our to do list, we know when we are leaving and can budget our time better, we can prepay and spread out our credit cards costs.

Renting a car for the one day would be very time consuming. You would have to taxi to the pickup point, drive, find the car drop off point, taxi to your hotel. Don’t know the cost.
Ferry port in Split is a short walk from the old part of the city. It is very convenient, especially if you are staying within the Palace walls. The arrival port in Dubrovnik is in Gruz, a short taxi ride to the old town. The tickets are fairly inexpensive.
We would rather take a relaxing boat ride than trying to drive.

We like to stay a minimum of 3 nights at any place to not feel rushed. Since you want to minimize your stops, I would suggest
Dubrovnik- 4 nights (one day to get over jet lag)
Hvar - 3 nights
Split - 4 nights
Dubrovnik- 1 night

If you do your research, you will find plenty to keep you busy in all these places. It will still be warm enough to swim also. Except for Hvar, we’ve been to Dubrovnik and Split a few times and always find new activities, places to see, and great restaurants. We took a really fun cooking class in Split.
If you want, I can recommend tour guides in Dubrovnik and Split for short walking tours or day trips.
We have never been to Hvar, we always go to Korčula. From my reading and experience, I believe Hvar has better beaches. Maybe someone else will comment about that.

I see that others are recommending visiting areas that are not easy to get to without a car, but you stated you wanted an easy trip. Adding more places with your short time frame will add stress. It’s impossible to see an entire country in just 11 days, especially one so spread out. If you enjoy your trip, another visit could be in the northern part.

Best way to spend 10 nights or more is to start in Dubrovnik ( 3 nights) then take a catamaran to Korcula (2-3 nights) then take a catamaran to Hvar (3-4 nights) then take a catamaran to Split ( 2-3 nights).

Nice daytrips include Lokrum National Park from Dubrovnik (15-minute ferry) Orebic and the Peljesac wine country from Korcula ( 15-minute ferry) ancient town of Stari Grad from
Hvar Town (30-minute bus); and ancient town of Trogir from Split ( 30-minute ferry/bus).

That is one of the most enjoyable itineraries I ‘ve ever done anywhere. Going by boat makes it a real relaxing holiday.

Have a great trip!

www.Ferryyhopper.com for boat times and fares.

We loved Dubrovnik, and we had no trouble filling our time there. We had 3 nights there, but that translated to nearly 4 days, because we arrived early in the morning and departed late afternoon. We spent an afternoon on Lokrum Island. Another day would have been too much, just because it would have cut into time elsewhere.

We spent two nights on Hvar, but we arrived late and left early, so that was one day. We found that to be enough.

We spent one night in Split, but we arrived very early, so it was a full day. We thought that was plenty, but another day here could work, too, as long as it doesn't limit your time somewhere else. If you stay longer, consider a day trip to Krka, if you aren't going to Plitvice.

(We also went to Zadar (2 nights) and Zagreb (2 nights) with a night at Plitvice in between.)

You can read my trip report here, in case you might find it helpful. https://community.ricksteves.com/travel-forum/trip-reports/croatia-and-ireland-may-2022-castles-and-forts-and-water-oh-my

We stayed here in Dubrovnik, and it was wonderful. https://www.airbnb.ca/rooms/5189080?source_impression_id=p3_1756218430_P3dVRFzE_HSwVL87 Ivana was a lovely host, allowing us to check in early (not guaranteed, of course, depending if others are booked in the night before), and the apartment was spacious and comfortable with gorgeous views.

We stayed at Hotel Podstine on Hvar, and we liked it: https://www.podstine.com

In Split, we stayed at Grgur Ninski Rooms. This hotel is small, but very well appointed and located. https://grgurninskirooms.com

Enjoy your trip! We loved Croatia.

We did a trip almost identical to what Kenko describes but in reverse and it was wonderful. The only thing we added was a day trip from Korčula to Mljet which I highly recommend especially if you enjoy some time in nature. The ferries between the islands are only a couple hours or less so it’s easy to see a lot in a short time.

Thanks everyone. All these suggestions help. We will certainly do at least Dubrovnik, Split and Hvar and possibly Korcula with 1 final night back in Dubrovnik for flight home. Just don't want to be too rushed. Sounds like the catamarans are fairly straightforward as well.
Dubrovnik 3
Hvar 3
Split 3
Dubrovnik 1 night pre flight home