Make Your Own Hide-A-Key Glue a rock to the top of a empty pill bottle, bury the bottle so that only the rock shows. A spare key is hidden inside! - 15 Secret Hiding Places That Will Fool Even the Smartest Burglar OR it could be a Geocache!
This is such a fun idea! or use dremel tool on a real brick. Perhaps a small real Lock & Lock could be used instead of a film canister. Would be watertight and more fun to be able to leave signature items and travelbugs.