Buy an Adventurers Pebble
The adventurers pebble is an artificial pebble that can go on its own adventures. Buy the pebble and leave it to be found.
When someone finds the pebble, they tap their phone on it and the pebbles message board is displayed. The person is then able to leave a short message on the message board before leaving it or moving it to a new location.
As the pebble owner you can leave a longer message that is shown at the top of the board and you can opt to get notified when a message has been left and the location of your pebble.