Santorini- There are some fun trips here. The tender will take you ashore, and there are lots of small shops selling tours of the area. The caldera tour is fun as you go to the volcano, climb to the top, then continue in the boat to a lagoon where you can swim to some warm springs. The whole trip is 3 hours and costs about $25 a person. Go to Oia, it is much nicer than Fira town and has fewer tourists. You can take the public bus or a taxi. That will easily fill a day.
Athens: Acropolis, agora, Plaka district. There is a metro that takes you from the port to the main town. Very easy to do. Just Google "Piraeus Metro" and you should be able to find the stop.
Corfu: You can rent a car and go all over the island. The town is fun to walk around. St. Spiro's church is pretty. I did a ship tour to Albania, which I would recommend.
Mykonos: I would go to a beach, here, as it is similar to Santorini.
Olympia: We rented a car and drove to Olympia from the port. Very easy to find, lots of signs, much cheaper than a ship tour. The ruins are self explanatory and have signs telling you what you are seeing.
Kusadasi: Ancient Ephesus! plus the Temple of Artemis, the museum in town is worth a look, and the Isa Bey Mosque is interesting. Search Ephesus tour cruise ship, and you will find a host of operators.
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