We took a Mediterranean cruise in May 2008 on Holland America. I imagine the shore excursions are similar. Regarding Naples, we took a 4-hour excursion which began in Naples, then included a bus ride to the lost city of Pompeii. The ruins of Pompeii are remakrable in that they are almost perfectly preserved in spite of being buried beneath tons of ash and lava from the eruption of Mt. Vesuvius in 79 A.D. We had a very knowledgeable guide and would highly recommend taking a guided tour of this area.
We also enjoyed our guided tour of Dubrovnik which included a performance of folk music and dancers. Very interesting! The only tour which I wouldn't recommend was "Lucca & Pisa." Much of the time we were on our own, not a lot to see and many high pressured sellers in the sourvenir stalls.
When in Barcelona be sure and try one of their tapas restaurants.