Last August Karin took me, Meg, and Juniper geocaching in the University of Washington Arboretum.
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Karin purchased the Geocaching app. An Intro version is also available.
We chose a shorter adventure. You use your iPhone to guide you via a map that uses the phone's GPS. There are also written clues to help you along your way. The app got us pretty close to where we needed to find the cache but in the end we resorted to using a handheld GPS device that was far more accurate than the iPhone's.
Eventually we located the cache, which we all signed and returned to its hiding place.
One of the tenets of geocaching is to leave nature in good condition and to carry out any trash you might find along the way. I found this nasty piece of trash to haul out.
Our introduction to geocaching by Karin, who had previously done a few other adventures, was a great time in the outdoors. I would recommend checking out the Intro version of the software, which explains how geocaching works and helps you (try to) locate geocaches in your area might pique your interest, which could justify the $9.99 price tag for the full version. I do think for more difficult geocaching you need a dedicated GPS unit that is more accurate; we used an iPhone 3GS to get us most of the way before using a Garamin device. I tried the free version in my neighborhood and found where the cache had once been hidden. Geocaching!