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

Play as Princess Corona
Successfully complete the game on 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.