Get Extra Hard Mode
Open the Very Hard mode by first picking your game and then in the Game Settings menu press and hold Z then press L, L, A, B. Or if you want to get it the hard way, defeat the entire game while on HARD mode.

High Speed Marathon Mode
At the Title screen, hold Z and press B, A, L, L. Now set the starting speed in marathon mode up to 99.

Bonus Trainers
At the Trainer Selection screen, hold L + R + Z on controller one and two.

Mewtwo's Stage
At the Trainer Selection screen, hold Z and press B, Up, L, B, A, Start, A, Up, R.

S-Hard Difficulty
At the Difficulty Selection screen, hold Z and press R, L, A, B.

Taunt opponent
Select any two player game mode. Have one player select a character, then repeatedly press A.

Badges
Defeat one of the following characters to earn the corresponding badge:

Character - Badge
Brock - Boulder
Misty - Cascade
Lt. Surge - Thunder
Koga - Sould
Erika - Rainbow
Sabrina - Marsh
Blaine - Volcano
Giovanni - Earth