Dubrovnik's a beautiful old city, almost completely intact, with many stores and restaurants to visit, as well a old churches and the great wall around it. Cavtat, a short distance away by car offers a quiet little town on the water with a few places to eat and shop.
After visiting wonderful old cities such as Kotor and Dubrovnik, Corfu was a disappointment. It was a true current city with two historical forts and all that modern cities have (something that many on the ship liked), but we found it less interesting than the medieval cities and a little dirtier. There wasn't too much to see in the old forts, but we did enjoy seeing how they were fortified-once here, definitely worth visiting.
We didn't take the excursion to Olympia so may not rate this port properly, but we were going to Athens and had been to Rome and Mexico, so didn't feel that more ruins would be that interesting. The port town was loaded with touristy shops, but nothing different and nothing worth seeing.
We found Santorini to be fascinating, with mules walking up the path to the city and through the town. And the beautiful colors of the houses and shops, the great water views, and many very good restaurants with decks overlooking the water made it one of the better stops. Plus many of the shops had unique items, not just the same tourist trinkets. We didn't take the tram but can see how great the views must be.
Athens, being one of the cradles of civilization, was great. The Acropolis was wonderful, the museum very nice and the surprise excavations throughout the Plaka area interesting. There are many virtually carless streets with tons of shops and restaurants everywhere. While other cities have a changing of the guards, the unique dress and march made this one very interesting.