Clothespin Math on a clothes hanger! LOVE this. Easy to make, and self explanatory. I'm thinking a wire clothes hanger would work better, so the clothes pins are easy to slide back and forth (almost like an abacus).
Math Concepts: This is an adding machine! It is super simple to make and fun for the kids to learn about adding/composing numbers. It will help those kids understand the equal sign in a way they haven't experienced before.