This is a sneaky way to hide keys inside an old piece of PVC pipe. (credit to Sean Ragan for the idea, just helping to distribute)
Step 1: Cut Your Pipe
Using a hacksaw (If you wish you can use powertools such as a table saw, but make sure you wear a good quality dust mask to protect against airborne particles) cut a length of PVC pipe to be long and shallow along the axis as shown in the picture.
Step 2: Position Your Pipe
Place your pipe somewhere in the place you wish to hide it. Ideally this should be somewhere in the dirt with a slight incline so that the smaller end can be placed up the slope, as well as being near a wall or similar large object to prevent people tripping on it and discovering it isn't a pipe. It helps to disguise the pipe if leaf litter and other normal things you'd find in your yard are covering it slightly.
Step 3: You're Done!
Anyone's images of the completed project, or steps taken while creating it with other tools than a hacksaw would be appreciated if they could be posted below. This is a simple instructable, but it's cheap and easy to do, as well as being useful. If you have anything you'd like me to change, feel free to comment.