In the Think Geek catalogue they had a hide-a-key holder that was a piece of pvc pipe with a sprinkler head attached. http://www.thinkgeek.com/gadgets/security/b7c9/ You take off the sprinkler head, put the key into the pipe, then put the whole thing in the ground in your lawn or planter area. Nobody would no the difference, other than it looks like a fake sprinkler head.
I thought that was a great idea but instead of spending the $4.99 on a fake one this is something that can easily be done using the real stuff. Depending on the size pipe you could put in keys, coins, or anything you could think of. If you put a real sprinkler head on to match the real ones it would be easy enough to plug it up so no liquid would get inside.
I do know of a crew who was working in the state park clearing trails. They found a metal pipe that was capped at both ends. They opened the cap and found 10 gold coins from the 1800s. Obviously this story wasn't advertised because the park personnel didn't want the general public to tear apart their park looking for more capped pipes.