Hide your spare key in a medicine bottle with a piece of a pinecone glued to the end. Then bury it.
Key Hiding Rock
9 Geocaching tools to keep in your pack (also add gloves, headlamp, and a stick/pokey thing to poke before sticking your hand in something, oh, and a grabber thingey mosquito repellant)
Florist Foam ... 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.