Pwll y Gwynt

The cave entrance sits tucked into the hillside near Crickhowell, and you'll need a torch to venture properly inside. Pwll y Gwynt - the name means "Pool of Wind" in Welsh - is a proper underground system that's worth exploring if you're already in the Brecon Beacons area, though it's not a polished tourist attraction with handrails and interpretive boards.

What you get is a genuine caving experience. There's a stream running through, limestone passages to duck through, and that peculiar coolness that caves always have even on a warm day. Most visitors manage the main chamber and a fair bit of the passage without specialist kit, though you'll want good footwear and something bright to hold. The walk down from the road takes fifteen minutes or so.

History types will appreciate that these caves have been occupied on and off for thousands of years - there's genuine archaeological interest here. Families with younger kids might find it a bit underwhelming unless they're keen cavers already; it's not Cheddar Gorge with gift shops and tours. Walkers doing the loop from Crickhowell, though, will find it a decent detour that actually rewards the scramble.

Cwmdu Campsite is your nearest option for a base, less than five miles away, so you could combine this with walking the surrounding moorland. Allow an hour for the cave itself, more if you're exploring the deeper sections. Bring a proper torch - phone lights won't cut it. Check conditions before you go; water levels rise after rain and it can get slippery.