Line up 21 buttons (or a different object you have a lot of). When it is your turn, you can take away 1, 2, or 3 buttons from the line. Continue taking turns. The person that picks the last button loses.
I found this game in Family Fun Magazine and it was a huge hit with B. It was interesting to see how he started to develop a strategy. If you give it a try, let me know how it goes!