Whatever you do, DO NOT do the excursion to St. Paul de Vence! A complete waste of time and money, unless you want to wander around a walled city that has row after row after row of so-called artist galleries, then get bussed back to the ship, with only a short view of one of the best beaches in the world. No real shops to speak of, either, just the endless array of galleries. (Santorini and Villefrance itself is MUCH better, IMHO).
Take the time to wander around Villefrance early in the morning on your own. It's safe, fun, and free. There is even a little tram that runs around for a few Euro. Then get a trip over to the beach in Nice. Only 15 minutes away, and beautiful. Plus you are close to where everyone is.
Contact me if you would like more information. We did the same cruise (Venice, Dubrovnik, Athens, Santorini, Naples, Rome, Villefrance, Barcelona) last August on the Millenium.
We've done 15 cruises, and that was absolutely the best cruise and the best way to see that part of Europe!