Deformed Psycho Racers : Hold L, R, and all the C-Buttons on the select vehicle screen.
New Paint Jobs : Press R or Z on the Acceleration/Max Speed bit after selecting vehicle to change the colours.
Examine your ship : On the Acceleration/Max Speed bit after selecting vehicle use the C-Buttons to look. You can also look at the course with them on the Select Course Screen.
See Captain Falcons Face : Complete any cup on Master Setting.
THE ULTIMATE CHEAT
On the Game Select Screen Press:-
L, Z, R, C-Up, C-Down, C-Left, C-Right, Start
I think that you will 'av a bit of a laff'.