Yoshi's Island DS Codes -

Time Trial mode:
Successfully complete the game, then load your cleared saved game file. The question mark that previously appeared un the lower right corner will be replaced with the "Time Trial" option. There are five extra levels (one from each World) in Time Trial mode that are playable with a black Yoshi.

Extra levels:
Get 100 points on each of the eight stages (except the secret stages) in a World for a total of 800 points. To do this, you must collect 20 red coins, 5 flowers, and 30 stars.

Extra 1 level: Get 800 points for World 1. 
Extra 2 level: Get 800 points for World 2. 
Extra 3 level: Get 800 points for World 3. 
Extra 4 level: Get 800 points for World 4. 
Extra 5 level: Get 800 points for World 5. 

Coin bonuses:
Collect all of the coins with a baby's face hidden in each of the following Worlds to unlock the advanced version of the corresponding mini-game.

Hard Bouncy Maze: World 3 
Hard Egg Toss: World 5 
Hard Flutter Challenge: World 2 
Hard Speed Eating: World 4 
Hard Tulip Shooter: World 1