I invented a new game (someone else might have come up with a similar game that I do not know of).
One side "hides" their king on a square by writing it.
The other player uses his King, Queen, two rooks, two bishops, and two knights to try to find the king.
The game is played similar to the game "Battleship".
When one person puts a queen on e4, the other player will say "hit" or "miss", based on whether or not the king is put in check.
There are a few other details to the game, but basically whoever gets more points after 3 rounds is the winner.
I was surprised that my students like to play this game so much. I definitely think this game helps players become stronger. I also showed this game to three very strong adult players and they enjoyed it as well.
If you are really interested in learning more about the game, email me.
Updated August 14, 2012
A few months after coming up with this game, Alexey Root put this in her book "Science, Math, Checkmate". This is the second time she was kind of enough to put me in her book! She called it "The Black Box". I think she didn't want to offend the good people of Hasbro or whoever makes the game "Battleship" but I'm not sure. Since then I haven't been playing it with students as much. Over time I've become more of a chess purist, and try to stay away from variants of chess too much. Although recently I "invented" a game that I called "Jeff Chess" because Jeff Feng and I happened to be at the camp where this became popular. Two sides balance a King on their head. Whoever resigns loses.