Treasure hunts were popular activities at our house, and several of the kids birthday parties featured a hunt for buried treasure. Usually a series of written clues was prepared before hand, and the guests were divided into two groups, or maybe just one, and given the first clue which would lead them to the location where the next clue was hidden. Sometimes there was even a map! When the treasure was found, sometimes buried in the garden and sometimes in another secret inside location, the stash of candy and trinkets would be divided among the participants.
Usually after such an activity, at least several of the kids and their friends would plan their own treasure hunt for other siblings or friends. Filling a box or other container with messages and important objects, then burying them as a "time capsule" was another favorite activity.
There have been several times when as a family activity we have written letters to our future selves, and sealed them up in an envelope to be opened in five or ten years. It is always interesting to open up the letter years later and to see the thoughts and goals and other predictions that had been written by your former self years before. In fact, I have one such envelope sitting on my desk right this very minute: