All Night Nippon Super Mario Bros. (FDS)
Worlds A - D
Each time you beat World 8-4, the game will add a star to the SMB logo on the title screen. When you have eight stars, at the title screen hold A and press Start. You will be whisked away to World A-1. (A maximum of 20 stars will fit on the logo, but only eight are needed to unlock Worlds A through D.)
To get fireworks at the end of a level, touch the flagpole to stop the timer when the ones digit of your coin total matches the ones digit of the timer (i.e. 34 coins, 194 time). If the digit matched is an odd number, you'll get three fireworks. If the digit matched is an even number, you'll get six fireworks. At 500 points per firework, this will really give your score a boost.
The magic number
To get a 1-Up at the end of a level, make sure your coin total is a multiple of 11 (00, 11, 22, etc.), and then touch the flagpole to stop the timer when ones digit of the timer matches the repeated number in your coin total (e.g. 44 coins, 194 time).
Invisible 1-Up Mushrooms
To activate a hidden block containing a 1-Up Mushroom in most x-1 levels, collect all the coins in the preceding x-3 level without dying. For example, collecting all the coins in World 2-3 unlocks the 1-Up block in World 3-1. You can also trigger the bonus 1-Up in the x-1 stage by warping to it.
A few things to note:
A few levels have coins sitting on blocks that contain power-ups. If you hit the block from below when the coin is still on top, you'll pocket the coin like usual, but the power-up won't come out, so be sure to grab the coin first if you really need the power-up.