Castlerigg one of those places you'll stop at for half an hour, tops, no matter how interested in ancient stones you might be. The day we went the wind was whipping around angrily so we stopped for about seven minutes.
It's just a ten-minute car ride from Keswick, if you choose to base yourself there, like I did.
Still, it's a MUST see in the lakes district and even cooler than Stonehenge. Set against a dramatic, stunning, hilly backdrop, this stone circle is thought to date to 3000 BC. which makes it about 5,000 years old.
Read a bit of backgrounder here but otherwise, just enjoy wandering around something incomprehensibly old. It's a spectacular sight.