I did this tour in September. I have a couple of tips and comments based on my experience. Greece is struggling financially at present and the majority of places we shopped at or ate at on our own preferred to have cash. Some refused credit cards all together. Paying cash also meant the prices were better for buying souvenirs. Be prepared to have cash on you if that is all they take. All the beaches were pebbles so take water shoes or you will have a hard time going in the water or walking along the waters edge.
Everything you see is amazing, there are however a lot of abandoned buildings and graffiti, a sign of the times there, so be prepared for that. No one experienced any pickpocket incidents though. Running on the Olympic track was a highlight for many people on my tour! If you are based at the Hera Hotel, there is an amazing view of the Acropolis especially at night. On Hydra, I highly recommend doing the long walk with the guide, it might be optional I can't remember but was a fantastic walk and we rented chairs and stayed at the beach (pebbly) for the day.
Safe and happy travels!