Been a geocacher for ten years now. Probably hold the world record (nine years) between signing up and finding my first geocache. But once I started, I was hooked. It's a lot of fun, finding the little boxes or tubes hidden away in interesting bits of the world.
The big rule for a geocache is that there must be something worth visiting - if the only reason to go to that spot is to find the geocache, it's not good enough. So there are historic markers, great views, parks, cultural objects. It's a fantastic way to see some of the locations in a city not covered by guidebooks.
I like leaving travel bugs - in my case, little Lego minifigs with a tag attached - and seeing how they go.
All over the world, it seems!
Great hobby for travellers. Or anyone, really.