Play as Luigi:
After completing the game, highlight a saved game at the file selection screen, then hold L R and press A.
Unlimited Mushroom House:
Complete the Star World to unlock the ability of repeatedly using a Mushroom House.
Infinite lives on Mushroom World level 1:
Go to the first Star Coin and stand on top of the stairs. Then, when the Noko Noko appears, go down to the second or third step. When the Noko Noko reaches you, jump (not stomp) on it. When done correctly, the Noko Noko will bounce off the wall but Mario will keep jumping on it. Note: This does not work as Golden Mario.