You can sign up here at http://www.geocaching.com/, a basic membership is free as is the iPhone app. There are literally thousands of caches waiting to be found. Located in rural and urban areas, you could easily break up road trips for a little treasure hunting!
We had our first go yesterday, it was a more urban cache and totally fun! The GPS took us straight to the cache site but it took us a while (about 30 mins) to find it as it was quite small and since it was our first go we weren't a 100% sure what it would look like.
In each cache there are also little trinkets, you can take one and replace it with another. In our first one we put in 2 old coins and took a glow in the dark star.
|A treasure found!|
|Gorgeous view on a very hot day|