Greece/Croatia/Italy

Indeed renting a car in one country and dropping in another usually incurs a hefty fee. Plus, that's an awful lot of ground to cover over two weeks. I think doing a round-trip loop from Athens and then flying to Dubrovnik is your best bet. Take it one piece at a time: what do you want to see in Greece specifically? To me, a week is enough for one or two island stops, and I am presuming you want a minimal time in Athens, but that will depend on arrival/departure times also.
Croatia is an exceedingly easy place to travel, and a week for Dubrovnik, Split, and Plitvice Lakes sounds about right.
September will be quite warm, so that should help with the packing--light weight clothing takes up less space. Just be sure to have a plan for doing laundry at some point.
ETA I am assuming also that you have already scoped out all flights, etc.

Each of your first two destinations needs a minimum of two weeks. If you go for one week to Greece, you can see Athens and one island. For Croatia, our favorites were the island of Korçula and Split.
If you want to see Greek culture, avoid Santorini, a tourist hot spot. Napflion on the Peloponnesean Peninsula is one of Greece’s most beautiful places. It is a two hour drive from Athens. .
And you don’t take a rental car across international borders without paying a huge drop fee( We were quoted 1000€ fee one time so dropped that idea) Also, you warn that you aren’t light packers so it will be much harder for you to move around easily. Slow down and enjoy your trip,
We each take a 22” suitcase whether for one or six weeks, even when we attended a wedding abroad.
So choose either Greece or Croatia, both are wonderful places to visit. And then fly to Italy for the wedding!

I agree with Suki. One country. And since it sounds like you already bought tickets to Athens, that means Greece.

I’m picturing you dragging big suitcases in and out of cars and ferries and hotels. so my second suggestion is find a minimal number of locations to settle in and do day trips from, minimizing your need to handle that luggage.

One idea is Crete. Find a flight to Chania, Crete, leaving Athens after 10 am on your arrival day. Fly to Chania, have a taxi take you to your hotel, and spend some time ( a week?), exploring the town and taking day trips by bus or taxi or rental car, as desired, without the need to change hotels & handle luggage. Full disclosure: I have not yet been to Crete but have been planning my trip for months & will spend my first week in Chania, also in September. Lots to see in Western Crete.

Then fly back to Athens, pick up a rental car at the Athens airport and visit Napflio for 3-4 days. Then back to Athens airport, turn in your car. And spend your remaining time in Athens.

There are dozens of other options but this would minimize location changes & give you a good overview of Greece.

You may have seen the "60 Minutes" segment on travel to Positano, but in case you didn't I wanted to caution you about driving there. During the tourist season it's adviseable to take a ferry instead because the road gets so clogged that it can take long hours to make your way there by car, and if there's an accident ahead of you, forget about it. Positano has become a victim of it's own popularity.

Thanks for all your help people. We really appreciate it. Despite your guidance against it, we are going to do both Greece and Croatia. We have to meet our Daughter in Dubrovnik, so that kind of fixes it. Here is what we are currently thinking, but would love commentary on if we are nuts, and also would love hotel recommendations.

9/12/22: arrive in Athens at 8:00am
9/13/22 - 9/15/22 tour Athens (3 nights)
9/15/22 - grab a boat to either Mykinos, Naxos, or Paros (looking for suggestions)
9/15/22 - 9/18/22 - enjoy the island (3 nights)
9/18/22 - boat to Santorini
9/18/22 - 9/21/22 enjoy Santorini (3 nights)
9/21/22 - Fly from Santorini to Dubrovnik
9/21/22 - 9/24/22 Enjoy Dubrovnik with our daughter (3 nights)
9/24/22 - Drive to Split
9/24/22 - 9/27/22 Enjoy Split and the waterfalls (3 nights)
9/27/22 - Drive back to Dobrovnik and catch our plane to Naples for the wedding

Would love your thoughts on this.

thx - Randy and Suzanne

9/27/22 - Drive back to Dobrovnik and catch our plane to Naples for the wedding
Would love your thoughts on this.

Presumably you are planning the easyjet flight at 250PM? ( That is my guess as it seems to be the only non-stop)

Just be sure to be familiar with their check in policy/timing and you sound like you will be checking luggage.

Be sure you build in enough time to drive to the airport, gas and drop off the car and complete all the check in activities in a timely manner.

My personal experience is that google maps is optimistic in their travel times

https://www.easyjet.com/en/help/booking-and-check-in/check-in

Thanks Joe,

We indeed are on the easyjet flight. Are we being too optimistic thinking we can drive from split to catch that flight?

Randy