Abalone

Abalone is like chess played with marbles but thankfully the rules are more simple and straightforward.  It is two-person game that bright seven year olds will be able to play.  There's even an Abalone world championship.

You have a hexagonal board with 14 white marbles set up against your opponent's 14 black ones. When it's your turn, you move one of your marbles, or a row of two or three marbles, one step in any direction. When your marbles meet your opponents, on your next move you can push them backwards, forwards or sideways.  The aim is to push your opponent's marbles off the board and into the gutter that surrounds it but your column of marbles has to bigger than theirs.  So you need two of your marbles to push one of their marbles, three of yours to push two of theirs. The winner is the first to push off 6 marbles.  For most people, the game will last about 20 minutes.

Here's a video explaining it: