We did that cruise on Princess almost 4 years ago; our itinerary included Rhodes in addition to your list. We had a fabulous time, but I will say, be prepared to see lots and lots of ancient ruins.

In terms of excursions, have you checked out www.cruisecritic.com ? That website is a terrific resource for sharing info with others on the same cruise.

We did a few ship excursions and were on our own in some ports.
Venice: Princess had a water taxi for a small extra charge, providing regular service between the ship and Piazza San Marco.
Dubrovnik: we took a taxi to the old town, then were on our own for walking the wall and touring.
Corfu: ship "excursion" - we chose a simple bus ride to the top of the hill by the old Venetian fort, and had an appointed time to be back for the return bus ride. Lots of nice outdoor cafes in a huge park, and we took a horse-drawn carriage ride around the old town.
Katakalon (for Olympia): ship excursion to the ruins of the original Olympics, quite a few miles from port.
Athens - on our own; did the Rick Steves walking tour (self guided). You can take a taxi or ride the subway to central (old) Athens.
Rhodes - on our own, walked the old town wall, and covered all of the old part of Rhodes. Easy walking distance from port.
Kusadasi: ship excursion to Ephesus in the a.m., on our own in the p.m.
Mykonos: didn't do a thing!
Santorini: on our own. There was 1.5 hour wait for the funicular from port to town. It's possible to walk (very steep) or ride a donkey up/down.
Naples: Hop On Hop Off bus tours of Naples in the a.m.; ship excursion to Herculaneum p.m.
Rome: we had spent 3 nights in Rome pre-cruise, so just went to the airport from the ship.

Truly a wonderful trip - can't recommend it highly enough!