Key to a Secret:
Bury a key somewhere they children will be playing, like a sand box, or a garden area where they are planting, or in the box of toys where they will be playing, etc. Hide clues that lead to a box that the key fits. Anything can be a treasure to children such as a cake mix that they can make, or movie tickets, or a new toy or treat.
For Younger Grandchildren a simple “hide the thimble type game where you tell them if they are getting warmer or colder as they move around the area of the hidden treasure. Then let them see something obvious that has a keyhole.