You can learn a lot of interesting information from taking a tour of a medical institution. Check to see if Hospital Santobono is open to the public while you're in the area.
You'll find Hospital Santobono in the commercial neighborhood of Arenella. It's free to enter, so you can leave your cash tucked away in your pocket. While you're in the area, why not also visit some other worthwhile sights, such as Castel dellOvo, Naples National Archaeological Museum and Castel Nuovo?