For 12+ kids who want a "we actually did something" day, the Vesuvius crater trail is a solid choice:
The walk from the car park to the crater rim is about 30–40 minutes each way, moderate difficulty, on gravel track — manageable for most kids in reasonable shape.
Tickets must be booked online in advance — they aren't sold at the gate, and there's no signal near the entrance, so buy before you head up.
Entry is by timed slot; in summer, arrive early morning to beat both the heat and the crowds.
A guide is included with your ticket and walks the group along the crater edge
Why not combine it with Pompeii or Herculaneum, both 20–30 minutes away, for a fuller day.
Information provided by NAPLES HIVE