Conkers is a traditional game played with chestnuts. To play each girl will need one or more chestnuts and a shoelace or other string. A hole is drilled though the middle of the chestnut (this can be an opportunity to teach the girls how to use a drill). Then the string is threaded though the hole and knotted at the bottom.
The girls play in pairs. One girl lets her conker dangle straight down while the other girl swings her conker to try and hit the stationary one. Then they switch. Traditionally, a point is scored if one conker splits the other. If you wish you could modify it so that a point is scored for each solid hit, but you should still allow for broken conkers.