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.
Decomposing Numbers: can do an activity with the number cubes. Students roll a number. Put that number of blocks in the big circle then decompose into 2 smaller numbers. Give students a recording sheet.
Clothespin Math on a clothes hanger! 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).