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Pompeji is a MUST, and Ercolano (Herculaneum) too. For the rest of the day you might climb up to the Vesuv for an overwhelming view of Napoli.
Capri is overcrowded with tourist.
OK, Sorrento is lovely. You can go there by train too (Circumvesuviana from Naples first to Sorrento, then back to Pompeji Scavi, thereafter with the train to Ercolano and than back to the cruise port.
Have a nice day in this aerea