Try & stay in Venice to see the Redentore festival & fireworks if you can.
Going to Olympia on my next cruise.
Santorini try to do Oia or Akrotiri. The island is spectacular & Oia is known to have one of the best sunsets in the world.
Ephesus you want to do the biblical sites as well if you can or a best of Ephesus.
Mykonos - we just went to a beach.
Athens - well you can’t not see the Acropolis. Plaka, Monisteraki. So much to see.
Chania- I would see what excursions are offered. Knossos, the 15km gorge, Spinalonga just off the top of my head.
Zakynthos- also know as Zante. I’ve not been.
Also look on the port of call boards.