Arduino forum member RickP made this “reverse geocache” from a French army medical box. I love the nixie tube display and the fact that it looks like something from a spy movie. The box only opens at the right GPS coordinates, and is trackable on geocaching.com.
He uses an Arduino, a GPS sensor, and a trio of nixie tubes, which indicate the box’s distance from that special spot. When the button is pressed in that spot, a servo motor unlatches the lid.
The large amount of power drawn by the nixie tubes had to be taken into careful consideration. With four D-cell batteries, Rick estimates that the box can do 600 GPS fixes before they run out. Connected to the power button is a latching relay, which allows the Arduino to shut the power off after the GPS fix has been attained.