Why not just keep your flexibility and let your plans evolve during the day. Its actually quite easy. I would suggest that you get off the ship, walk over to the hydrofoil/ferry pier (next to the ship towards your left as you exit the pier) and take a hydrofoil over to Capri (about a 40 min ride). If you find that you really like Capri (we love the place) than spend the day on the island and take the hydrofoil back to Naples. If, however, you find yourself bored with Capri, just take the jetboat (same pier as where you docked from Naples) over to Sorrento. From Sorrento, you can take the train (its the Circumvesuviania Line) back to Naples (about an hour for the train and 10 min for a taxi to the ship or its a 25 minute walk. Keep in mind that most of the decent stores in Sorrento will close around 1 pm until about 4. On Capri, some of the high-end boutiques also close in the afternoon, but many places remain open all day.