You put this ridiculous helmet over your head that has these 3D TV goggles, and when you move your head, it's like you're actually moving your head in the game.
Here's how to do something like that for under 100 bucks.
Camcorder with Viewfinder and video input
Gyro mouse (More on that later)
Normal wireless mouse (optional)
Step 1: About the Gyro Mouse
I got mine for 40 bucks at Circuit City. I've been looking around online, and that was a really good deal. You can use any kind of gyro mouse, but it needs to be able to do these things:
Pointer is based on tilting and rotating, not actually pointing at the screen. AKA no sensor bars or things like that.
It needs to be wireless.
If you don't have one try to find one here.
If you can't find one, this instructable will be impossible.