Any trip to the Campania area must include a visit to Pompeii and Herculaneum. Both were covered when Mount Vesuvius erupted in AD 79.
Mr B and I went to both on our visit and it was amazing! At both you can hire a guide to take you around (make sure to use one with a badge). There are audioguides that can be hired or you can purchase a book from the shop.
Pompeii and Herculaneum can be reached from Naples on the Circumvesuviana train. Go to Pompeii Scavi/Villa dei Misteri and Erclano for Herculaneum. You can get tickets that will let you into both.
I suggest going to Herculanuem first thing because it is smaller, then you will not be rushed to leave Pompeii if you are not ready. Herculaneum could take 2-3 hours. If you go here first, then you would also allow the crowds to go down at Pompeii. It is the most visited place in Italy so you find yourself waiting to see things. Also Herculaneum is more intact whereas many things from Pompeii are in the Museo Archeologico Nazionale in Naples which is worth the visit to have a complete trip.
Pompeii was amazing and everyone should visit if you are in this area. It is huge though!
Herculaneum is only 4.5 acres versus Pompeii's 44 acres so it can be daunting trying to see everything. A guide would be very helpful.
I liked this place better because there were more things intact. It was smaller so you did not feel like you missed anything and it was less busy.
It is closer to Vesuvius so many of the items were left more intact than at Pompeii. In fact you can see the bones from the 300 people who died in the boat sheds trying to escape.