As a UT alumna and former President of the UT Math Club, I recently had the opportunity to participate in a STEM Expo about Girls in STEM. It was so fun to talk with girls about the opportunities for women in Science, Technology, Engineering and Math.
We introduced the girls who attended to fractals, Hanoi towers, Mobius strips, Rubik's cubes and other math related games. I'd recommend any of these as great tools for boys and girls to explore interesting, hands-on math concepts.
If you're interested in teaching the girls in your life more about Women in Mathematics, the American Mathematical Society has a great site with information and resources: