Hidden Vehicle:
To access the hidden vehicle called the Demon, go to the select game screen (Main Menu) and press the following combination of buttons: Up, Down, L1, L2, Select then Start. If the cheat code has been entered correctly you will hear a sound effect. The hidden vehicle will be available in all game modes.

All Arena & Tracks:
To access all the Arenas and Tracks go to the select game screen (Main Menu) and press the following combination of buttons: Down, Up, R2, R1, Start then Select. If the cheat code has been entered correctly you will hear a sound effect. All the Arenas and Tracks will be available in Time Trial and Battle Mode games.
See ya guys

Thanks to Revolution reader Brett Hoffman!