Saltstone

Drive down towards Kingsbridge and you'll spot Saltstone - a proper geological curiosity that sits right on the South Devon coast. It's basically a massive chunk of sandstone that's been weathered into something resembling a fortress, all rust-coloured and dramatic against the sea. You can get right up to it depending on the tide, which is when it's worth the visit. The thing is, you're not spending hours here - it's a five-minute stop, maybe ten if you're the type to scramble about looking for fossils. Walkers will appreciate it as a waypoint on the coastal path rather than a destination in itself. Families with younger kids won't find much to occupy them beyond twenty minutes, but older children who are into geology or just want a slightly weird photo will get a kick from it. The nearest town, Kingsbridge, is close enough that you could combine this with proper exploring inland - the estuary walks around there are genuinely excellent. If you're camping at Wilton Farm, Karrageen, or any of the other spots dotted around the area, Saltstone works as a quick beach-break leg-stretch rather than the main event. Go when the tide's out if you can, and check the weather beforehand - it's exposed and the wind can be bracing. Free, obviously - it's just a bit of rock. Worth the detour if you're already exploring the coast, but don't make a special trip.