This game is one my children loved, once they learned to read...While they were unaware, I would take pieces of paper, on which I would write "clues" that rhymed. These clues would be placed in locations where they would find the next clue. Eventually they would find the last clue, where I would have some sort of treat or fun surprise.

Par example, the first clue might read,

"Go to the box with a red flag that moves
there if you're smart, you will find the next clue!"

When they would figure out it was the mailbox, they would run like crazy to get their next clue. It could be much simpler, for example,

"Go to the place where you always feel better
when you open it up and discover a letter."

Now that I am a grandmother, they still ask if I will continue the tradition.

for non-readers, you could use pictures, and guide them through the process.