Key Keeper

Purchase an inexpensive kid's lock and key set and put the key on a lanyard or a coiled key holder bracelet or simply make a necklace with string or ribbon. Attach the lock to something of significance for the child and let him have responsibility of locking and unlocking it and keeping the key safe. Kids may like locking up the carrying case for their toy cars, their doll tote, or some other container of prized possessions. Be sure that you have a way to open the lock in case the key is lost. To avoid losing the key, choose a designated spot (such as a peg on the wall) that serves as a key holder when the key is not being worn by your child. Praise your child for being responsible with the key over a period of time.

Picture from: http://gadgetnation.net
Activity from: The Playskool Toddler's Busy Play Book by Robin McClure (2007)