Napoli Centrale has a beautiful big Cafe' off the lobby on the main floor, they sell yummy pizza by the slice, all varieties, and I think the most delicious "American" coffee. (For me, they got out a big cup and poured whatever they had in the pot they were holding, and it was delicious!). They also sell a lot of other food. Huge space, lots of tables and chairs. (There is a McDonald's next to the cafe', if you walk thru the cafe', you'll see it.)
There is a luggage storage room down a long hallway, ask one of the many cops around for directions, cost was 6 Euro a bag plus show your passports. Or You can keep your luggage with you at the cafe', adults and kids begging for money may approach you so keep your money in a neck wallet or money belt, i recommend no purses, and stay with your bags at all time, it's Napoli, they've been doing this for decades. When floor crew see them, they escort them out.
Take a cab to the Ferry, walking in Napoli is different, you will have motorcycles zinging past you, coming and going, on both sides. They go up on sidewalks too, not supposed to, but they do. You don't have much time, many eating places close between 3 pm and 7 pm, so why not just relax in the cafe'.