Nathan Pryor of Vancouver, WA, built this counting box for his son.
My 4-year-old son loves counting and numbers, so I built him a dedicated counting box. Select a value from 1-10 with the rotary switch in the middle, then push the big green arcade button to add that to the total shown on the LED display, or push the red button to subtract it. It’s driven by an ATMega, and the box itself is made from laser-cut bamboo and acrylic. A viewing window on the back lets him peek at the circuit inside. The site has a write-up of the building process.
I have to also give props to Nathan for his awesome enclosure, he has an interesting part of his post where he talks about designing it and sending it out for lasering.