Go to options, then go to the nickname option. Enter these codes at the nickname screen. WARNING: Use of any of these codes deactivates saving for as long as they are in effect.

Password - Effect

AAARGH - Enables all hazards and pickups in time trial
STONEME - Rockets turn non-invulnerable opponents into stone
IMACOPTER - Top-down view
ROCKETMAN - Unlimited dumb rockets
SPINACH - Unlimited superpower energy
GIMMIE - Unlock everything