How to see the best of Campi Flegrei—The Phlegrean Fields in just a daytrip from Naples
The Phlegrean Fields are insanely easy to explore on public transportation.
Just take the metro from Naples to Pozzuoli, Sophia Loren's hometown, where you can see an ancient Roman amphitheater (» more). Then catch the local bus up the hill to wander around the caldera of the active Solfatara volcano—still venting gasses and steam, the ground hot to the touch, yet somehow creepily tamed, with a campground in the middle of it and everything. (» more)
Ride the bus back down to the center of Pozzuoli where you take an early lunch overlooking the Temple of Serapide on the main square, broken columns that slowly rise and fall along with the surrounding landscape (You don't see it actually happening; it moves on the scale of decades, but you can see the effects).
In late afternoon, when the light shines best through the rock windows of the ancient keyhole-shaped gallery, consult the Cumean Sybil in her cave—one of the foremost soothsayers in ancient times—then climb to the top of the hill for a late afternoon panorama of the bay. (» more)
Grab the bus from Cuma back to Pozzuoli, which will pass under the AD 1st-century brick Arco Felice mountain pass, then on the left the round Lago d'Averno, the lake that was the gateway to the underworld where Aeneas descended to confront Hades.