Just had an e-mail from Martin Fussen to tell me about PLACESPOTTING: a site that allows you to create your own map based quizzes...
Go to the website and click CREATE.
Move and zoom the map until it shows an interesting place. Provide some hints for the person / friend who will try to solve the puzzle (a number of hints can be provided) and a message that they will see if the riddle is solved. After saving the quiz you can send it by mail to a friend or embed it in your homepage as a picture. The task is to find the place shown in your picture on a second google map.
The website has 6638 different quizzes stored. 108274 quizzes were solved and
Looks worth checking out and creating a few quizzes.