Something else to hook up and get working with the Arduino is cool, even though Gameboy cameras are getting more difficult to find.
Something else you might try (which has already been done as well, so there are some examples out there) is hooking up and reading an optical mouse sensor. Resolution and capture speed tends to be really terrible (32x32 or 64x64. at about 1FPS if you are lucky), but they are fairly easy to interface with, and easy to source, too.