A clue must be placed at the bottom of a container which is filled with water. The treasure hunters must drink the water to empty the container so that they can get the clue.
This challenge has a good scope for variation. Obviously you should alter the quantity of water based on the number (and size!) of the hunters - one to two large glass per hunter is plenty; make sure it is something achieveable that will not make your hunters ill. You could make the challenge more difficult by changing the water for something less appetising to drink: tomato juice isn't often popular with children, neither is water with peppermint flavouring in it.
Bear in mind one of our top treasure hunt idea tips: don't let the hunters get stuck! If they look like they can't finish the task then have an alternative task ready.
This treasure hunt idea is courtesy of Tim in Brecksville, Ohio, US. Thanks Tim!