Double health/speed in Vs. Mode:
While you are at the title screen press Down, Down, Up, Down, Left, Right, Down, Up, Up, Up, Up, Right, Left, Square.

Extra Vs. Mode characters:
While at the character select screen press L1, L1, Down, Down, R2, Select, R2, Select, R1, L1, R2, R1, R1, Select.

Big characters at world map:
While you are at the title screen press Left, Square, Up, Circle, Right, Triangle, Down, Triangle.

Vs. Mode Elephant Pool Arena:
Win 20 team battles.

Vs. Mode Metal Age Arena:
Win 100 arena battles.