Paestum

Paestum is located south of Naples and is easy to reach with public transport, making for a great day (or half-day) trip from Naples. This Greco-Roman archaeological site is home to the remains of three ancient Greek temples and a museum.
Founded as a Greek colony in the 6th century BC, Paestum still has defensive walls, a Roman forum, what's left of a Roman amphitheater, and a collection of ancient tombs, all of which can be explored at your leisure!