Mini Racers
Select Mario Kart GP Mode. Then at the Character Selection screen, hold Y and press A. Or, highlight a character and press A + B + X + Y.
Special Cup Tracks
Select Time Trial Mode, choose a racer and press Left to highlight the Mushroom Cup. Now press L, R, L, L, R, R, A.
Turbo Start
As the last light starts to dim, hold B.
Reset All Saved Data
At the Title screen, press L + R + Y + A.
Reset Race Times
At the chosen time trial, press L + R + Y + A.