There are many creative ideas that can also serve an important purpose; the following is one of them. If you often lock yourself out of your house, this do it yourself project may just be for you.
If you have old pill bottles lying around, this is where they will come into use. You will need an old pill bottle, a clean rock, and a small amount of epoxy glue. Epoxy glue is best for this particular project, as it is not sensitive to extreme temperatures, and this craft is going to be placed outside long term and could very well be exposed to all sorts of extreme weather.
First, take the label off of the bottle, and clean it out, making sure that the rock can feasibly fit securely on the lid of the bottle. Then, put some glue on the rock and the lid, and allow each of them individually to sit for about 2 minutes. After this time has passed, stick them together. Let the glue dry for about 24 to 48 hours, and then place the craft outside.
At this point, you can put your key into the bottle, and then find the ideal spot in your yard to put this entire thing. You can easily hide this among other rocks or in a garden, using other items to help disguise this if you would prefer. Potential intruders and prowlers will not be able to figure out that under this one particular rock is a key.