While the game is paused press X+Y+Z to obtain a clean pause screen (No menu options etc).

Hold X+Y+Z whilst selecting the track to activate Mirror Mode (Track reversed).

Jeffry Dance - On the Expert course, as you approach the Jeffry statue, Press the X button rapiddly so he turns, and stands on his head.
Mirrored Tracks - Hold X+Y+Z and choose a track.
No CPU Cars - First highlight a track and then hold X + L.
Race with Uma - When at the Main Menu, hold L + X + Z, then press C.
Use the Daytona super car - Hold R + X + Y + Z, then press C to choose a game mode on the main menu. 'The Hornet' will have changed, it will have 5 stars in every catigory.
Use the Horse - Hold L + X + Z, then press C to choose a game mode on the main menu.
Use the Second Horse - Hold L + R + Y + Z, then press A to choose a game mode on the main menu.