Some people like to stash a spare key to their house in a fake rock or under a potted plant. Another good way to hide a key is to build a small PVC pipe with a cap and coupler for a waterproof storage area that will look like an innocent access point to a drain.
PVC Workshop suggests using a 10cm long piece of pipe with a threaded cap and couplers to mimic the appearance of an access point. Just assemble the pipe, add your keys and bury it in the soil. Alternatively, you can just bury the entire PVC pipe -- but you'll have to remember where it is and dig it up when you need the key.
Key Hide Away [PVC Workshop]