Dr. Mario 64 (N64)

S-Hard difficulty for Story mode
In Story mode, highlight the Hard difficulty setting and press the Z Button. You should hear a sound effect, and the box will change to read "S-Hard." Press the Z Button again to change it back to Hard.
Harder computer opponents
In Vs. Computer mode, hold the L Button and press A to change the difficulty to Hard, and hold the L Button and press B to change the difficulty to S-Hard. If you perform the codes while actually selecting your opponent, one or two stars will be added underneath their picture for Hard and S-Hard difficulties, respectively.
Unlock Metal Mario
In Story mode, play as Mario on Normal (or higher) difficultly. Reach and beat Rudy without losing once, and then you'll fight Metal Mario. Beat him without losing to unlock him.
Unlock Vampire Wario
In Story mode, play as Wario on Normal (or higher) difficultly. Reach and beat Rudy without losing once, and then you'll fight Vampire Wario. Beat him without losing to unlock him.