Collect one million coins to unlock a title screen featuring a gold Mario and coins.
Coin Rush mode
Complete the Castle level in World 1.
Hatless Mario
Reach "Triple Crown" status by getting 1,110 lives.
Mushroom World level 1: Unlimited lives
Go to the first Star Coin then stand on top of the stairs. When the Noko Noko appears, go down to the second or third step. When the Noko Noko reaches you, jump (not stomp) on it. If done correctly the Noko Noko will bounce off the wall but Mario will keep jumping on it. Note: This will not work as Golden Mario.
Play as Luigi in single player mode
Successfully complete the game. Then, highlight a saved game at the file selection screen hold L + R and press A.
Rainbow levels
Jump on the flagpole at the end of the indicated World when the last two digits of the timer displays the listed number. The original NES sound of a world being cleared, fireworks, and a rainbow will confirm success. Return to the world map, and an entrance to the corresponding rainbow level will appear at the entry point of each world.
World 1: Timer ends in "11"
World 2: Timer ends in "22"
World 3: Timer ends in "33"
World 4: Timer ends in "44"
World 5: Timer ends in "55"
World 6: Timer ends in "66"
Flower World : Timer ends in "88"
Mushroom World: Timer ends in "77"
Star World: Timer ends in "99"
Save File Stars
Your save file can be adorned by as many as five stars, provided you meet the corresponding requirements (as indicated below).
Beat world 6
Gather all star coins in worlds 1-6, plus the Mushroom World and Flower World
Gather all moon coins in the Star World
Access all of the game's alternate paths (including those you must pay to unlock)
Obtain 1110 lives, which appears as three crowns
Don't use the golden leaf (which appears if you die within a stage too often) or you won't get to enjoy sparkling stars and that would be sad indeed.
Star World
After you defeat Bowser, you can spend 90 star coins to unlock the special Star World.
Triple Crown
In past Super Mario Bros. titles, crowns would replace digits once you collected a certain number of lives. That's true in New Super Mario Bros. 2, as well. The requirements are different, though. You'll see the first crown replace a number once you acquire 1000 lives, a second crown when you reach 1100 lives, and a third one when your tally reaches 1110 lives. When you have three crowns, Mario and Luigi will remove their hats as a special bonus (they'll put them back on if you lose any lives from there, though, until you once again acquire a third crown).
Unlimited Lives
Work your way through Course 1 in Mushroom World, until you reach the stairs near the first star coin. Stand on the second or third step and wait for the descending enemy to reach you. Jump lightly to land on it and send its shell toward the wall. The shell will bounce back and Mario will continue to jump on it, racking up points and eventually earning you extra lives. This should be a familiar trick if you played the original Super Mario Bros. on NES. It won't work in New Super Mario Bros. 2 if you try to perform the trick as Golden Mario, though (your enemy's shell will eventually disappear instead of continuing to grant extra lives).
Unlimited Mushroom House use
Complete the Star World.
Unlimited Mushroom House Visits
If you thoroughly complete the game, you'll finally be able to visit Mushroom House locations as often as you like. You need to complete every standard and extra level (including cannon levels and Star World stages), find every alternate exit, and collect every moon coin in the Star World. When you do so, the mushroom house spaces will turn blue to signify that you can visit them as often as you like.