Win water buggy race mini-game:
Intentionally lose the water buggy race mini-game, then immediately press
R + A + B when the screen fades to white. Your water buggy can go faster 
during the next attempt.

Princess Corona:
Successfully complete the game under any difficulty setting. Return to the title 
screen after the ending sequence. Load the previously completed saved game to 
unlock Princess Corona. 

Doraemon message:
Unplug all controllers and turn on the Nintendo 64. Doraemon will appear and say 
a warning at the opening screen.