This is just off the main road from Naples to Pozzuoli, about 40 minutes out on the 152 bus route (get off when you smell sulphur). It’s a bit flat compared to Vesuvius, but a lot easier to reach if you want a volcanic experience and don’t have much time (or energy) to spare. It’s privately owned, so you pay four euro for admission, but in return you get a small map of the more interesting features, everything is clearly signposted (including examples of the local flora and fauna), and there are all the usual facilities (loos, café, gift-shop). There’s also a campsite inside the crater, which must make an interesting alternative to beach and countryside locations.
The inside of the volcano itself looks a bit like an exhausted chalk-pit-- slippery white clay surfaces, enlivened by bursts of steam coming out of every available crevice, and a low, evil-looking lake bubbling away in the centre. Anything genuinely dangerous is well signed and fenced off, but there are occasional deserted buildings (the Victorians believed it was healthy to breathe in the fumes) to provide points of interest, and you can wander around and touch and sniff to your heart’s content.
If you are wearing any silver jewelry, keep it well covered--the sulphur in the air will turn it glossy brown in minutes.