Everyone loves unlockables. They make a game more fun and give you something to work for. Here are the best unlockables you'll find from Super Mario Bros.
Unlockable Character - To play as Luigi, press L and R, then press A to select a save file you have of the game. You'll then control Luigi instead of Mario.
Toad Houses - The toad houses give you different items based on their color. To control the color of your next toad house you'll want to end the current level with the last two digits on the timer the same. Have two 1's, 2's, or 3's will cause a red toad house. Red houses will give you items. A pair of 4's, 5's, or 6's earns a green house with a mini-game. This gives you a chance at an extra life. Finally, 7's, 8's, and 9's are an orange toad house and home of the Mega Mushroom.
Beating the Game - There are a number of treats for those who beat the game. Once you've beaten the game, you'll be able to buy four different patterns for the background of the game. If you've cleared all the levels and all the bonus levels, then the toad houses will now be available to you with unlimited items. You can also save anywhere in the game. This is a great help for playing through a second time.
Of course, there are always things you'll have to do the old fashioned way. To unlock Worlds 4 and 7, you'll have to meet special requirements. For World 4, you'll want to go fight the last boss, in the castle, of World 2. But before you do, keep in mind that you'll have to be playing as mini-Mario when you beat him or it won't work. For World 7, you're doing the same thing. Beat the last boss in the castle of World 5 as mini-Mario and World 7 will be available.