I read about this guy who made an interesting locking box for a wedding present. It is a locked box that will only open if is in a certain spot on the Earth. It has a built in GPS and when you press the button it will tell you on the screen how far away you are from the location of it opening. It also only allows you to press the button 50 times. If you would like to read more about it here is a link to the guys blog.
http://arduiniana.org/projects/the-reverse-geo-cache-puzzle/