Enter the following at the Main Menu. You'll hear the word "Yeah" to confirm entry:

Play as Sarah:

Left, Left, Square, Right, Right, Circle, R1, R1

Play as Mick:

Left, Right, Circle, Square, R2, L1, L2, R1

Play as Bonobo:

Right, Circle, Left, Left, Circle, Circle, Square, Square

Open all courses:

Right, Circle, Square, Left, Square, Circle, R1, L1

All boards:

Right, Right, R1, R2, Left, Left, L1, L2

Play as Shao:
Beat the game twice as Frankie to enable Shao.

Easier qualification:
During play, pause the game and press Right, Left, Right, Right, Up, Down, Left, Circle, Triangle . This will make it easier to qualify.

Opening the gates:
Beat all three stages with a character and two new gates for the previous stages will be opened. You must beat the game with all four normal characters to open every gate, allowing access to every area of every stage.

New Boards:
Beating the game once with each of the four bonus characters opens up five new board designs per character.

Mirror Courses/Time of Day:
Beat the game a second time with all 4 bonus characters to get mirrored tracks and the option to turn night into day for the three stages.