Naples has the BEST day trips of any Italian city. Pompeii (40 min). Capri (50 min ferry). Amalfi Coast (90 min). Herculaneum (20 min). Caserta Royal Palace (40 min). Paestum Greek temples (1h). The concentration of world-class sites within 1 hour of Naples is unmatched by any city on Earth.
Under 1 hour (the essential 5): 1. Pompeii (Circumvesuviana train 35 min, €2.80 — a Roman city frozen in 79 AD. Pre-book €16. Allow 3h. The most visited archaeological site in Italy). 2. Herculaneum (Circumvesuviana 20 min, €2.20 — smaller, BETTER PRESERVED than Pompeii. Organic material survives — wood, fabric, food. History nerds prefer this). 3. Capri (ferry from Beverello port, 50 min, €20-25 — Blue Grotto, Faraglioni, Villa Jovis, Piazzetta. Go early, return by 6pm). 4. Caserta (train 40 min, €4 — 1,200 rooms, 120-hectare garden, cascading fountains. Italian Versailles. €14). 5. Vesuvius hike (bus from Ercolano Circumvesuviana, 20 min + 30 min walk to crater rim, €10 — look into an active volcano, the Bay of Naples below).
1-2 hours: 6. Amalfi Coast (SITA bus from Sorrento 1h30, or ferry from Naples to Positano 1h40 — Positano + Amalfi + Ravello in 1 day is TIGHT but possible by organized tour. GYG day tours €60-90). 7. Procida (ferry 40 min, €12 — pastel houses, Marina Corricella, Il Postino. The QUIETEST island in the bay). 8. Ischia (ferry 1h, €15 — Negombo/Poseidon thermal gardens, Castello Aragonese, Maronti beach). 9. Sorrento (Circumvesuviana 1h, €4 — lemon groves, cliff views, delizia al limone, base for Amalfi). 10. Paestum (train 1h, €6 — 3 Greek temples from 5th century BC, better preserved than anything in Athens. + buffalo mozzarella farm nearby).
11. Campi Flegrei (Metro Line 2, 30 min — Pozzuoli amphitheatre, Solfatara boiling mud, Baia underwater archaeological park. Walk on a supervolcano). 12. Castel dell'Ovo + Mergellina (not a "day trip" but: walk the Naples seafront — Castel dell'Ovo fortress on its islet, Via Caracciolo lungomare, Mergellina harbor. The Naples most tourists miss — the waterfront that faces Vesuvius across the bay).