All of them. You can get a train to katalon to the first olympic statium, a bus to knososis palace, you can then pay something like 10 euros for a tour. If you get up earily enough you can get a bus all the way over to lindos to look at the ruins there and still have time for the beach . both santarini and mykinos i got busess very easily to beaches which cost my daughter and I a couple of euros each way. we went in july-aug this year. The only trip I did was athens and it was well workth doing this for the tour information. often i either found someone on the ship to pair up with for taxis etc or i found someone outside to share a taxi with. we went for 2 weeks and it was great.

Very easy to DIY - Try google and 'cruise ports call boards' - the first result will give you all the info under the Europe section

crete, mykonos,santorini, rhodes, corfu, Athens and Dubrovnic. All other places during the two weeks cruise need more thought as to how to get to the sights so i recomend an excursion for the other places on the cruise. Enjoy !