Terry Olson and the Orange County Arts & Cultural Affairs have put together an interesting event around public at and geocaching:
"Find clues from this website to hidden caches near various sculptures and public art objects around the county. In each box is a journal where you can leave your mark and the date you found it. There is also a stamp that you can use to stamp your own journal or "public art" passport. After you have written in the journal, and drawn something or stamped it, put the cache back where you found it so the next person can have the pleasure of finding it and putting in their mark."