Something I some times dabble with.
I got a Geko 201 for Christmas a couple of years ago and not long after I read an article in one of the broadsheets where they posted the geocaching.com web site. Initially I started looking for caches to learn how to use the device but it soon developed into a way of keeping my son entertained. We'd spend sunny afternoons trekking back and forward over bits of west London parkland hunting carefully hidden Tupperware treasure troves. And having a GPS with me gave an accurate tally of the number of miles we were walking in our searches.
If you have kids, a GPS, and find that throwing a Frisbee along a footpath has lost it's edge in getting them walking then try and see if you can get them out caching.