The Reggia di Caserta is the largest royal palace in Europe by volume. It makes Versailles look like a starter home. 30 minutes from Naples by a €3.50 train.
Multiple trains from Napoli Centrale. The Reggia is literally across the street from Caserta station.
Regional train, 30 min, €3.50. Spend morning in the palace, afternoon in the gardens. Easiest day trip in Campania.