Doraemon 3: Nobita no Machi SOS!

Faster Water Buggy

If you happen to lose the Water Buggy race, do the following just when the screen turns white: press A and B and R at the same time. Your speed will be increased.

Play as Princess Corona

Beat the game on any difficulty level. After you've watched the ending, go back to the title screen and select your Cleared Game Data save spot. You will now be able to play as Princess Corona.

Dame da yo!

Here's a hidden Easter Egg: if you start the game without having any controllers plugged into your console, Doraemon will say "dame da yo!"