I actually didn’t even realize that there was a second city that was wiped out by the Vesuvius eruption that you could visit — Herculaneum, which is decidedly smaller than Pompeii, but definitely worth a visit.
Obviously if you’re in Naples, you’ve gotta go to Pompeii — it’s a quick, cheap train ride away from the city, and it’s an interesting enough piece of history to be common knowledge around the world. You’d be crazy to miss it.