Greece recommendations

Rick Steves has a book on Mediterranean Cruise Ports that includes all the stops you mentioned. That might be a good place to start and then come back here if you have specific questions. You can order on Amazon or possibly get at your local library.

Yes I am aware of the RS Mediterranean cruise book. I have taken a look and didn’t feel it was that helpful.
Am hoping that I can get some insight from experiences.
Thanks

Athens is a great place to visit with a lot of sites such as the Acropolis, Agora, etc. in the central part of the city that you can walk to see. When on beautiful Santorini you'll want to wander around Oia and Fira a bit. I'd also suggest renting a car and driving around the island some - Akrotiri and Red Beach are great stops essentially adjacent to each other and once you get off the rim of the caldera it's less touristy and has a more "greek island" feel.

I can't help with Santorini or Mykonos except I went on a day trip to Mykonos as part of the Delos/Mykonos cruise. Too crowded and expensive for me but others like it.

I always spend 2-3 days in Athens whenever in Greece

My favorite spots are:

Central Markets on Athinas St.
Anafiotika on the side of the Acropolis. It resembles a Greek village in the Cyclades.
Kerimikos Cemetery, the ancient cemetery in a quiet and peaceful area of Athens.

Lots more so check out this website:

https://www.athensguide.com/

Start with these. Trip Advisor is a valuable research tool.

Santorini: https://www.tripadvisor.com/Attractions-g189433-Activities-Santorini_Cyclades_South_Aegean.html

Mykonos: https://www.tripadvisor.com/Attractions-g189430-Activities-Mykonos_Cyclades_South_Aegean.html

Athens: https://www.tripadvisor.com/Attractions-g189400-Activities-Athens_Attica.html

While in Santorini you want to watch the sunset and hike to Oia. According to my sister, in Mykonos you want to see the windmills. Rick Steves GR Athens & the Peloponnese guidebook has a self-guided walking tour that will take you to them. If you buy it make sure it’s his sixth edition.

As for the Ephesus part: If you can fit it in (and on a day trip you may not be able to), you may want to consider a visit to the town of Selçuk, about 2 miles away. It has a number of historic sites itself, and a fine museum; and it's a rather pleasant little city in its own right. (My visit was in Dec. 2016.)

We spent a very pleasant day on our own in Ephesus on a cruise. It filled the entire day more or less for us. There is a lot to see, especially if you purchase an additional admission not included in the entry price. We took a taxi. Our experience: We were appalled at the extraordinary fares the drivers were asking and kept walking away from the ship. The further we walked the cheaper the fares became; I asked as we passed the lined up cabs. When the fare bottomed out we took a cab. The driver arranged to drop us at the entrance and meet us at the exit at the time we agreed upon. We don’t often take taxis but this was the only time we ever had a problem. Despite having carefully, repeatedly and agreeably establishing the price before getting in the cab he now demanded a higher fare than we had arranged as he dropped us off. His appeal was really pretty pitiful, so much so that in exasperation I simply gave him a “for heaven’s sake is that the best you have?” look, handed him the amount we agreed upon and walked away. That was it. A nice day overall. On Mykonos we spent a big part of the day on Delos, easily and quickly reached by boat. Delos is a large archeological site of significant historical and mythological interest. Safe travels.

In Athens you want to visit the Acropolis and its museum. My next trip to Europe will be to GR and I’m also going to try to see the Ancient Agora and the National Archaeological Museum.