Hey Ben,
With the upcoming Dev Kit of the Oculus Rift (http://www.kickstarter.com/projects/1523379957/oculus-rift-step-into-the-game) I am looking forward to programming some truly immersive content.. not just for games but other areas as well.
So my idea is the build an device that one could step into/onto and keep the user slightly elevated off the floor, similar to like and elliptical machine but a way to secure your self to it so you can't step off or fall off, and have the motion of the device be fed back into the PC as the forward or backward momentum.
This would be used for a form of exercise. Like Running around a block in Rome, or and where else without having to leave your home.
