At the game selection screen, press Up, Down, Up, Down, L1, L2, L1, L2 to unlock all games.
While in the Rainbow play room, press Circle to change how the effect works. It is somewhat inversed and is interesting to see.
Put your entire hand over the camera's lens.
Walk left and right in front of the camera in hard mode to wash a lot of windows and get a very high score.
Get a high score on Boogie Down to unlock more tracks.
Get some kind of stick or pole and hit the enemies with that. Do not keep swinging, but see what you are actually hitting.
Take a remote control from a television or other device use it on the Eye Toy. You will see the infrared light that is normally invisible to the naked eye.
When playing the Boxing game on any difficulty setting in the Japanese version of the game, either get a good score or knock out the robot. After the match, the trainer who helps you will say "You'll make that robot...", but the on-screen subtitles clearly read "That'll make that robot..."