Get Cheat Menu
Hold L, C Right, C Down, and the A button at the main menu.

Big Head mode
Enter BLAHBLAH at the cheat menu

Stunt Bonus
Enter SHOWOFF at the cheat menu.

Turn all riders invisible
Enter INVISRIDER at the cheat menu.

Unlock Hill Climb
Finish 1st in the Gold Round of the Amateur Season.

Unlock Soccer
Finish 1st in the Silver Round of the Novice Season.

Unlock NES Excitebike
Finish the Tutorial Mode by completing all the stunts.

Unlock 3D NES Excitebike
Finish 1st in the Challenge Round of Pro Season.

Night Mode
Type "MIDNIGHT" at the cheat code entry.

Mirror Mode
Enter YADAYADA as a code.

Small head mode
Enter PINHEAD as a code.