I've been working on a cardboard and hot-glue coin-operated machine, inspired by Tim Hunkin, of course, and the first step was figuring out how to make something coin-operated.
Tim has instructions for a coin-operated switch that works really well, and you can make it out of cardboard.
Tim has instructions for controlling the switch with a relay, but you can also control it with an Arduino. Arduinos are great, because they give you a lot of control over the behavior of your machine (Luckily, I was looking for an excuse to brush up on my Arduino skills). I started by programming the switch to turn on an LED, but it's easy to change the LED out for a motor, and then you really can control just about anything . . .
So with a little bit of code, a low-torque microswitch, and something fun to charge for, I'll be in business!