I would bury gold, silver, pennies, and nickels in separate caches. By the time they are
looked for there may be some electronic long range locators that are user friendly or one
of the grandkids may be an expert dowser. Then they can try on one item at a time.
Also some family tree info or family biographies would be nice find along with family
pictures. Maybe a copy of the deed to the farm and a current newpaper. Also maybe
an old rifle or revolver, they may be outlawed by that time, if you want them to have
one, then they will. Maybe some hermetically sealed seeds from the current crop off the
farm. Just suggestions, one of them may interest you.
Seems like a good fun idea to me.
Rich