Enter the following at the Enter Password screen. You will then have to get all the flags on each level. Down, Y, Right, A, Right, Up, Right, B, A, Down, Start.
Free for All Mode
Enter the following at the Enter Password screen. Neither player will be it. Up, Y, A, Right, Up, Y, B, Down, Right, Down.
Grid Racing Mode
Enter the following at the Enter Password screen and then choose a two player game. At the world select screen you'll find three new worlds where you may race. A, Y, Right, X, Right, Up, Down, Y, Right, Down.
See the Ending Credits
Access the 'Enter Password' screen, enter the password A, Y, A, Right, Down, Y, Up, Right, Right, A and press Start.
All Flags Mode...