Tried out geocaching earlier this year and discovered it's a lot of fun. You can sign up for an account for free, and if you really like it they have a "premium membership" for $30/year. The advantage to paying is you can download files with the locations and put them on your GPS instead of programming in coordinates by hand. I was surprised to put in my local zip code and see all of the caches in the area. Apparently this is a popular hobby!