Check out MSC cruises. We just got back and it was a great ship and a great itinary.
Venice (should stay a couple of days...magical place)
Mykinos..great taverna called Niko's.
Athens..must go to acropolis. But, we got our own taxi from Piraus and it was a hassle to get a good price. Go on a tour in Athens.
Corfu...I didn't know anything about it, but it was the best of all.
Dubrovnik..very pretty old city. But I'd already lost my heart to the Greek Isles.