Naples is a very fascinating city. There is different ways of visiting it. The National Archaeological museum may complete your Pompei/Herculaneum visit. Combine that with a drive through the scenic sites of Posillipo, Mergellina, overlooking the bay at the castles. Or consider walking through the old part of town and the narrow alleys trying some of the local street food (pizza, sfogliatella, babà, neapolitan caffè,...). If you are adventurous you may consider exploring the Underground Napoli.
There is a lot in the city, so it is up to you to pick your type of tour here.