Dubrovnik we did our own thing and went to the wrld heritage site, really expensive if you take the ships 'dubrovnik express', get a taxi and agree a price before you get in and youll save loads.
Corfu - on our own. Just walked around Corfu.
Cephalonia - Best of Cephalonia with ship reall good.
But would have gone on the glass bottom boat tour had t not been sold out by the time we got on the ship.
Katakolon - Worth doing Olympia i think as there is nothing in Katakolon just a few shops on a small street and thats it.
Athens - Best of Athens ship tour. Really good takes you to acropolis and an olympic stadium.
Really hope you enjoy your cruise.
Do your own thing - cheaper and you avoid being herded around and forced into shops! Check out ports of call section of cruise critic for ideas.