Obtaining the N-Space Card Game
In order to unlock the N-Space Card Game, you will need to obtain a minimum of 80,000 points in single-player mode. You must also successfully clear the stage for which you gained the 80,000 points. Once you have done so, go to the main world map in order to unlock the N-Space Card Game.
Defeating Bowser Without Damage
During your fight with the final boss Bowser, make sure to use little Mario (meaning no mushrooms or flowers). Since Bowser can only harm you using the top part of his body, little Mario is too small to get hurt. You can avoid taking seriously damage this way and defeat Bowser even faster.
Transforming a Hammer Bros. into a Treasure Ship
You need to collect coins in any multiple of 11, have the tens digit in your score match that multiple of 11, and end the stage with an even number on your timer. For example, you can have 33 coins, 1030 points and end the stage with 142 seconds. You will then see a Hammer Bros. be turned into a Treasure Ship full of coins. Be careful as this cheat only works in worlds 1, 3, 5 and 6.