Guitar Freaks Codes -

Battle mode:
Press Pick, Green, Green, Pick at the mode selection screen.

FF mode:
Press Red, Green, Blue, Pick(2) at the mode selection screen. Fast Forward mode increases the speed of the game.

Hidden mode:
Press Red, Blue, Green, Blue, Red, Green at the mode selection screen. Notes will disappear early in this mode.

Random mode:
Press Blue, Green(2), Red, Green, Pick at the mode selection screen. Some notes will be randomized.