The University of Florida Physics Department has a lobby exhibition which to me is very reminiscent of the Discovery Gallery in the London Science Museum. One of their latest exhibits is a Pendulum Wave Machine. They used the GPIO and animation facilities of Pi Presents to provide tightly coupled user interaction with the machine. You can see the machine here
John from the Physics Department describes how it works. It runs Pi Presents Gapless version 1.3 on a Raspberry Pi 2 board. The application has a main slide show that runs non-stop until a guest presses the start button. Starting triggers a sub-show slide show that also controls the state of the machine so it stays in sync with the show. We use three GPIO pins for switches, one to start the machine by viewers and trigger the sub-show off of the main show, and two micro switches that tell us the position of the machine (it’s state). We also use one GPIO pin for driving the electrical motor that operates the machine.