When visiting Naples, it is worth the 25km trip south to the ruins of Pompeii (you will sometimes see the modern spelling of Pompei with only one 'i' but the Roman spelling is usually retained for the excavation site).

Plaster casts were taken of victims in the positions they were found
The restoration work is ongoing, but it is evident from what can be seen that it was a thriving town until disaster struck. In 79AD nearby Mount Vesuvius erupted, burying the town under a layer of ash up to 6 metres thick.
The nature of the event meant there were many victims who died suddenly, with their shapes preserved in the ash at the moment of death - the lack of air and moisture below the layer has helped to keep these intact and the excavation team used plaster casts to retain the shapes.
Many of the images are disturbing, but the fascination of the town is that it gives a good insight into Roman life at that time. There are murals, paved streets, sewage systems, houses, and even brothels intact (you can tell the brothel, apparently, by the oversized phallus symbol in the front of the building).

This indicated a brothel was inside
There were many erotic and sexually explicit artifacts taken away and hidden for hundreds of years after being discovered. These can now be viewed in the
Naples National Archaeological Museum (an Italian language site by default, but you can select English).
Pompeii is a day trip from Naples and you would need a full day if you also plan to take in the nearby town of Herculaneum, which suffered a similar fate to Pompeii. As with many tourist sites, there are a variety of ways to get there and a variety of costs, depending on whether you do it as part of a tour, roll up on your own, book online before you leave home, etc. It is cheaper (half the price) if you are a European citizen aged between 18 and 25.
Check out the official
web site for gate entry fees, or go through a travel agency.

Some of the exhibits are disturbing

Nice setting for a town, shame about the volcano
From November to March the ruins are open from 8:30 in the morning and close at 5:00pm, but the last entry is at 3:30pm.From April to October the closing and last entry rimes are two and a half hours later. The site is only closed a few days each year (New Years Day, Christmas Day and 1st of May).