Eynsford Castle

The ruins sprawl across a grassy slope beside the River Darent, and what strikes you first is how compact it all is - you're standing in the remains of a Norman keep in the heart of a Kent village, not tucked away in the Cotswolds. Eynsford Castle dates from the 1100s, and the flint stone walls still give you a proper sense of medieval life, even if half of it's crumbled away. You can walk right up to the keep, peer into the rooms where someone once lived, and the river running past adds a rather nice touch.

It's genuinely free and takes about 45 minutes if you're not rushing. Historians will find it rewarding - the layout's clear enough to understand what happened here over nine centuries. Families with young kids will get 20 minutes of poking about before they're ready for something else. The site's unmanned, so go when you like; early morning or late afternoon beats midday crowds, though it's never heaving.

If you're based at Thriftwood Country Park, about five miles away near Sevenoaks, this makes a sensible half-day sortie. The village itself has a couple of pubs and a reasonable chip shop. The castle's not going to dominate your holiday, but it's the kind of place that costs nothing and teaches you something, which in modern Kent is frankly worth the short drive. Bring decent shoes - the ground can be boggy after rain.