Nathan made this "Counting Box" for his son -- I love LED displays set into hand-crafted wood boxes, and the simplicity of this toy makes it a nice blend of traditional toymaking craftsmanship and high-tech electronics:
My son—now four, but three when I conceived this project—loves numbers and counting. More than a few times I’ve peeked in at night to find him asleep with a calculator in one hand and a flashlight in the other. And one of his favorite things to do with the calculator is incrementing “1 + = = = = = = = = = =” until he can’t keep his eyes open any more. I decided to build him a dedicated machine that would do nothing but count up and count down.The Counting Box (via Make)
My initial concept was simple: a seven segment LED display so it could be read in the dark, a rotary switch in the middle to choose the value to operate with, and big buttons for adding and subtracting. Green makes the number get larger and red makes the number get smaller.