Prefabricate a long narrow clear tube with a watertight seal at both ends. Bore a large hole in the side at one end, large enough to pass a key through, attached to a similar sized piece of wood or balsa wood. Attach the tube firmly to an upright tree trunk or post which cannot be moved, so that the hole in the tube is at the top and the key/object is visible at the bottom.
Challenge players to remove the key without removing or breaking the tube.
If you want you can make it that the keys are required to unlock something as part of second challenge or sequence of challenges.
– Answer is to fill the tube with water/liquid and float the key to the hole, but they may find another way.