We did Athens, Santorini and Crete. All were great in different ways.
Athens was much better than we expected. We really enjoyed it. Of course, part of this was because we stayed at the Royal Britagne, with views of the Acropolis. I highly recommend this place. We did quite a bit of walking and really loved the city (especially the semi-licensed packs of well kept, stray dogs). The rooftop restaurant at the Britagne is beautiful for breakfast, but I would not recommend it for dinner (over priced and underwhelming).
Santorni was gorgeous and I look forward to going back. But, again, it has a lot do with where you stay. I think a poster up thread recommended staying up away from Fira, in one of the smaller villages overlooking the Caldera We stayed in Imerovigli at the Chromata and it was perfect. Quite, cave style hotel with perfect views of the Caldera. The village around us was nice and quite, with a couple of nice places to eat. We walked into Fira (about a 15 minute walk, although it is a hard uphill coming back) a few times for dinner and nightlife. We took a bus to Oia. What a shitshow. Yes, it has some iconic views, but that is where the cruise ships land. We could barely walk through some of the streets and we were there in "shoulder" season / October. I can't imagine what it would be like in Summer. I would not stay there and I would not even go back there to visit. Fira would not be as bad. Oh, and as recommended above, definitely do a Catarman cruise. You can do a day one with more beach activity. We chose a sunset one and still got some swimming in (especially at the hot springs) and it was a highlight of the trip. Pick a nice one where there are going to just be a handful of people on board with you.
Crete was quite nice. I can't say too much about it because we stayed at a big hotel (Blue Palace) that was full of surly Russians. The villages nearby were nice. We skipped meals at the hotel to eat along the water in more simple tavernas. Much better. We were in Elounda and visited Spinalonga island, which was creepy and interesting. The beach was rocky. But I liked the vibe in Crete a lot. It felt like Mexico. Knossos was a good stop to see some significant ruins on the way to the airport in Heraklion (one of the most ridiculous aiports I have been to, by the way).