Yesterday I discussed the Digi-Comp I, an educational toy that was a mechanical computer. The same company followed up with what was probably a more fun version, the Digi-Comp II. In this version, marbles rolled down a ramp, and depending on how you set different gates, they would roll to the left or right, flipping switches as they went, leading to the output of a mathematical problem. A bit like the game Plinko on the Price is Right, but the balls aren't just bouncing randomly. Here's a good explanation on a giant version. Brdavis built a LEGO version.
Here you can see it in action with his explanation.