Cheat code screen:
Hold L + C-Right + C-Down and press A at the main menu to display the cheat 
code screen. 

Invisible riders:
Enter INVISRIDER as a code at the cheat screen. 

Big Head mode:
Enter BLAHBLAH as a code at the cheat screen. 

Stunt bonus:
Enter SHOWOFF as a code at the cheat screen. 

Mirror mode:
Enter YADAYADA as a code at the cheat screen. 

Night mode:
Enter MIDNIGHT as a code at the cheat screen. 

Unlock all stunts:
Enter TRICKSTER as a code at the cheat screen. 

Classic Excitebike mini-game:
Successfully complete tutorial mode to play the NES version of Excitebike 
under "Special Tracks". 

Soccer mode:
Get first place in the silver round of the novice season to unlock soccer 
mode with two or four players. 

Hill climb mode:
Get first place in the gold round of the amateur season. 

Excite 3D mode:
Get first place in the challenge round of the pro season. 

Temporary invincibility:
Pop a wheelie immediately after the race begins and fall off your bike. The 
CPU will reset your biker, giving you temporary invincibility.