I used to do letterboxing which is a similar but much older hobby on Dartmoor - great fun and I still prefer it over geocaching! You get a rubber stamp in each letterbox you take an impression of and build up a collection over time, some are very cool, there's more per area (look under a "suspicious" rock on Dartmoor and chances are there might be a letterbox there) and it uses a map and compass rather than a GPS so it requires more good old fashioned orienteering skill.
Still, I can't get over to Dartmoor all that often because of distance - geocaching is thus great because there's loads nearby and it is similar
Found 58 so far, hopefully getting to 100 soon! (I'm also berry120 on there if anyone's wondering.)