When I was a kid, we had this event at my elementary school where we wrote the title of our favorite book on a piece of paper with the school's address on it and tied that paper onto the string of a helium balloon. Whoever found the balloon was supposed to mail the paper back to the school and we had a map where we marked where all the balloons were found.
Well John and Yoko did something like that in 1968. During one of John and Yoko's art shows, on July 1st, they released 365 white balloons into the air. Each one had a note attached that said if found to return to Robert Fraiser's gallery in London in care of John Lennon.
It would have been pretty awesome to find a white John and Yoko balloon caught in the tree of your front yard. And if you actually sent the card back in, look at what you got! A note signed by John and a pin from the art event--pretty cool prizes for just finding a white balloon!
Nowadays people still do balloon releases for various events, but really they are terrible for the environment because animals choke on the deflated balloons and they litter the ground. But still---it is a neat concept.