At the Bike Selection screen, press Up, Left, Up, Down, Up, Right, Left, Right, Circle.
All Styles
At the Style Selection screen, press Left, Up, Right, Down, Left, Down, Right, Up, Left, Circle.
Slim Jim
At the Rider Selection screen, press Down, Down, Left, Right, Up, Up, Circle.
More points
Just when the time is about to run out, press X to bunnyhop and hold Down and X or Up and X and you will continue to get points even after the time has expired.
Extra Bike & Character
Beat the game with any character to get Slim Jim and a bonus bike.