Add an island??

My first choice would be an island. I don't know much about Poros, I only spent a few hours there on a day trip so can't comment on that. If not an island, I would opt for Nafplio. We stayed in Delphi for one night and that was fine and it is in a lovely area so 3 nights wouldn't be too much, but I much preferred Nafplio on the coast.

I would give Napflion or Chania more nights.
We did a day trip from Napflion to the island of Spetses. We drove to a water taxi, very easy and interesting. Very green , a big contrast to the Cyclades which I assume you already have visited. The views from Delphi down on olive groves that go to the sea is beautiful.

Thank you so much. We debated trying to drive to Meteora but were concerned that driving would take up too much of our time and leave us too little to really enjoy the monasteries. Perhaps another time...

It's a lot of travel, so I would also add more time to existing stops. Nafplio for sure. Maybe somewhere close to Olympia? Messene was stunning.

I go to Greece every year and always visit at least one of the Saronic islands as they are so close and convenient to Athens. My favourite is Poros (better beaches in my opinion) but Agistri is also very nice. From Poros there are ferries to Galatas on the Peloponese, and from Galatas - buses to Nafplio so it may fit better in your itinerary.

Many thanks for your suggestions. We finally decided to expend our days at several stop to really enjoy the ancient sites and to then head to Crete to recover. We SO hope we can do this... Safe travels everyone.

Day 1  pick up rental car at airport drive to Galaxidi/ 3+hours Ganimede Hotel 4 nights

En route - Hosios Loukas Monastery and Distomo
Day 2 Delphi 30 mins from Galaxidi. And Amfissa/Vagonetto
Castle of Anmfissa and Archaeological Museum or Fokis Mining Park
Day 3 Wander Galaxidi - Nautical and Historical Museum, Aghios Nikolaos church, Aghia Paraskevi church
Day 4 Leivadia or Chaironea and/or Orchomenos and Arachova
Day 5 Olympia. 3 hours drive Bacchus Inn 2 nites
Visit Olympic stadium, temple of Zeus, Archaeological Museum and/or Archimedes Museum
Day 6 Klio's honey farm, and Olympia Gi winery
Day 7 Kardamnyli 2 1/2 hrs from Olympia Hotel Liakoto. 3 nites

Day 8 Visit Ancient Messine

Day 9 Day off - Beach (Stoupa) or Pool
Day 10 Nafplio 2 1/2 hr drive Perivoli Country Hotel & Retreat or Hotel Ippolito 4 nites
RS walking tour. Visit the Archaeological Museum, National Gallery.Walk the waterfront.
Day 11 Mycenae
Day 12 Tiryns and Epidaurus
Day 13 Mystras
Day 14  Return to Athens, drop off rental car fly to Crete/Arrive Heraklion
Heraklion GDM Megaron Hotel 3 nites
Day 15 Heraklion’s Archaeological Museum and Knossos
Day 16 Old Town Heraklion, Fort of Koules, Historical Museum of Crete
Day 17-22 Chania - a beach vacation Ammos Hotel 5 nites
Day 22 Depart

I assume you have looked at maps for driving distances, but I would consider trying to see both Messene and Mystras en route between destinations. The drive down into Kardamyli is very long, so you may want to avoid backtracking when you can. We saw Mystras as a day trip from there, but only because it was a rainy day (we had planned to see it in between Nafplio and Kardamyli). When you are on the main highway, the driving is quick, but once you are on the secondary roads, it slows considerably. Sounds like an amazing trip!

Thank you - your note is very helpful. I did try to calculate driving times using google maps - but obviously the times I found were very under-estimated!! Looks like I need to do much more homework!

That was just my first impression, so you might be totally fine--just something to double check. I did find Google Maps estimates fairly accurate, maybe just round up a tad. If you are an early riser and traveling when days are long, it can definitely be efficient to get up, drive to a site for a few hours (taking a picnic can facilitate that), then carry on to another destination. I would not miss Messene or Mystras, probably our favorite two sites.
I'm so happy to relive that trip and can't wait to return to Greece!

Thank you again. I will double check - and, for sure, Mistra and Mycenae are on the list!! Some day, we will be able to travel again...

Try ViaMichelin for driving-time estimates as well. Still, everyone warns of the need to add a buffer. The estimates don't include stopping, traffic tie-ups, getting lost, looking for parking, etc.

We stayed overnight in Sparta to visit Mystras after not being able to do it from Napflion as a day trip( illness.)