Greek dilemma

May be too late , or you have other reasons, but being that you have to be in Athens to start the cruise, I would have put the Athens days at the end. That way you do not have to 'get back' to Athens for the cruise boat.

An alternative is to see Olympia and continue on to Athens that evening/night, drop car at airport and stay at a nearby hotel for an AM flight to Santorini. To be within 50 miles of Olympia and skip it, somehow seems wrong.

Are you fascinated by ancient ruins (not “oh, it’s just another pile of rocks”)? Much of the Archaea Olympia site needs some imagination to picture how magnificent it truly was, so long ago. But it’s still a historic, amazing place. And the museum there is really a treat. We had a guided tour that included the museum back in 2018, and we stayed afterwards and went through the museum a second time, to appreciate and soak in everything there.

If you skip Olympia, there’s Ancient Sparta to break up the drive back to Athens.

The 15th through the 18th overall give me anxiety--do you really want to drive that much, with four one-nighters in a row?
I know some people like road trips, so I am just asking you to answer that for yourself, no right or wrong answer.
I would not consider missing Olympia any worse than missing Mystras, Messene, or any of the other sites. Something has to give.

I don't know if this will help you in your decision making or make it worse. We didn't visit Olympia or Messsene, but we did visit Mystras last May. IMO, Mystras is outstanding! It is beautiful! Consists of Upper Town and Lower Town with castle ruins and many churches that are intact and filled with ancient wall paintings and floor mosaics. I loved it, and I would love to return. Which I don't say about too many places, even though I have thoroughly enjoyed every place we visited. But generally we like to visit new places. But Mystras is one place I hope I can return to.

I visited Olympia twice and found it fascinating both times. The ruins are much more than a pile of rocks. It was a thrill to stand at the starting line in the stadium where ancient runners started their races. And the museum is top notch, with what may be an original statue by Praxiteles--not a copy. It would be a shame not to see it.

We were thrilled when we arrived at the stadium in Olympia! It was not difficult to envision what went on there. Walking by the statues of shame
was amazing and a good lesson for today. Some had told us not to visit Olympia but my Greek friend said we had to go. She was right! And to consider using an extra day for Santorini instead of visiting Olympia would be a huge mistake! No comparison.

When I went to Ollympia one year, end of May, I arrived 4:30 in afternoon, checked into small hotel ... the village is nothing reallly... just 2 long streets leading to the ancient site, with hotels and cafes. About 7 or 8 pm, lots of busses arrived with tour groups going into hotels. at Breakfast I got best tip of all -- Hotelier said the tour crowds usually start outside in ruins so I should go first to Museum.... at 8:15 it was nearly empty!! only 4 people. Delightful to wander alone. I'd studied the art in a seminar ... the Sculptures were amazing. One glassed-in exhibit was riveting; hundreds of bronze miniature helmets (left as tokens of tirbute in Zeus Temple) all facing same way... like a ghost army. As I left the museums hundreds from tour groups pouring in. Of course many crowds outside around ruins, but it's a huge site, so not crowded.

A good map & guidebook is v. important, and if u r not in a guided tour, u need background so it doesn't look like piles of rocks. NOTE: one can also see Museum crowd-free in late afternoon. A shivery note when leaving; I walked on a bridge over a stream that was labeled "Alph." This is the sacred river of Legend -- and in fact, is the reason that the stunning art of Olympia survived (without being looted in 19th C by British aristocrats as so many sites were).... The Alph flooded about 1500 years ago and buried the entire site in mud until modern times, when Greece was able to protect the art & artifacts against foreign ransacking.

Could you skip Stoupa and travel via Sparta to a place like Pylos instead?

An alternative to Olympia and flying under the radar is the spectacular archeological site of Messini.

Not commercialized, fewer tourists and in a beautiful rural area.

Well laid out, lots still standing, few tourists and a lot closer than driving to Olympia.