I can't give any RCL specific advice because my experience has been with another line. I'm about to have my first Royal cruise myself.
But I can tell you what I've learned about the excursions myself.
Provence - if at all possible, see Nice and Monte Carlo. Cannes is nice, don't get me wrong. But there is so much history in Nice, the market is unforgettable. And if you can get into Monaco, you have access to some pretty amazing stuff. It should only be a 90 inute ride I would think. Very worth it.
Livorno-Skip Pisa. I know how instinctive it is to want to see the leaning tower. But once you have spent 2 hours standing there looking at a bell tower you will realize you've missed about 2000 years worth of history in Florence which is an infinitely more interesting city, more shopping, more culture, more art, and more to talk about.
Rome-you can't make a mistake in Rome. No matter where your tour takes you it will be amazing. If I had only one day I would make sure I chose the Colosseum/Ruins, the Vatican, and some shopping along Via Condotti below the Spanish Steps.
Venice is my favorite city on the planet. It's a shame Venice isn't your disembark city because I could recommend an exquisite hotel just off the Rialto bridge. But anyway, see St. Mark's square of course, take the gondola ride (don't let anyone scare you off because it's "too expensive". Everything is expensive in Venice. So what. The city might be under water one day and you'll wish you'd done it.) And make sure and take a taxi ride along the entire length of the Grand Canal.
Corfu-When we took our Greek Isles cruise I almost slept through the Corfu stop because I thought it would be a take-or-leaver. BOY was I wrong. There are sections on this breathtakingly beautiful island where the roads are actually tiled with gorgeous slate. The streets look like an expensive kitchen floor. The shopping is unbeatable.
Never been to Croatia. Would love to.
Athens-it amazes me how many people discourage buying the wares from the people in the street. Yes, the hand made linen tablecloths are over priced. And yes, if you say no to the first lady there will be 20 behind her selling the same table cloths. And yes......you will be absolutely delighted with it and show it to everyone when you buy one anyway and bring it home. Ignore the pessimists who view street merchants as some kind of urchin. They're poor and can use the work. But moreoever, if you can find this kind of art and culture in cities near you at home lucky you. The only thing we have being sold in the street here in Detroit will get you a 5 to 10 year felony. I say "lighten up!" to those pessimists. Cruising around Europe is a time to splurge, celebrate and smile. Not be a cynic.