I've just read this post at @dalmatiasecond blog and I really love the experience he shares: a riddle hunt based on popular children books.
There's also a wiki where you can share your riddle hunt http://riddlehunts.wikispaces.com/
Here the post explaining the proposal:
The Birth of Riddle Hunts (A Hit With Kids): "The idea of clues to lead kids on a scavenger hunt started with Ron Clark. If they started with someone before him, sorry I didn't know! I a..."