I just love these kind of hidden secret things. But my favorite is the safest safe in the world.
When it comes to hiding things, this can be done in many different ways. Camouflage or concealment are the most common ways to hide something and in more sophisticated situations cryptography can be used to hide the content of a message. Hiding money is also common but sometimes can lead to very large problems when discovered (or if you forget where your money is hidden).
Rather than the old standard under-the-doormat or inside a plastic rock, this fake inground sprinkler head offers a good place to put a set of spare keys or maybe an emergency USB flash drive. It looks like a regular sprinkler head and the tough water-tight plastic case holds multiple keys.
More info: Sprinkler Key Secret …. Pssst!