Pittsburgh hackerspace HackPGH used leftover helium from their LEAD Balloon project to facilitate another project: the Aerial Ping Pong Drop. They filled biodegradable party balloons and launched origami butterflies and ping pong balls into the atmosphere. What for, you ask? They used Egg-Bots to write a URL on each ball, so that when it was found, the finder could enter his or her latitude and longitude into a form, which populates a Google map.
We'll assume you're ok with this, but you can opt-out if you wish.