Just a quick tip today from one of my earlier game design classes (we actually came up with this as a group in the middle of class while critiquing a student project):
If you're designing a paper prototype for a digital game and one of the core mechanics involves accurate positioning (examples include aiming in an FPS, positioning a paddle in a ball-and-paddle game, or maneuvering an avatar through an obstacle course), flicking a coin on a flat surface towards a target gives a reasonable approximation.
In our particular case, the student was designing a ball-and-paddle game. We discovered that if the coin was the paddle and a single flick was your allotted movement (then you repositioned the ball), it had the realistic property that it was much easier to hit a ball coming right at you than one that was landing halfway across the screen.
For an excellent example of this mechanic in action, see the non-digital game Pitch Car. Okay, so it's not Gran Turismo... but it could easily be the basis for a non-digital version of Mario Kart.