Unlockables

Character Coins Bonus - In the game there is a coin with one of the baby's faces on it in each level and if you manage to find all of the coins in each world you unlock the harder version of a minigame in the minigames menu!

Bouncy Maze (Hard) Get all character coins in World 3
Egg Toss (Hard) Get all character coins in World 5
Flutter Challenge (Hard) Get all character coins in World 2
Speed Eating (Hard) Get all character coins in World 4
Tulip Shooter (Hard) Get all character coins in World 1

Secret BonusChallenge Panels - At the end of each level there is a ring that can have 0 to 5 flowers on it depending on how many flowers you found in the level. When you jump in this ring there will be an imaginary ball that circles around the ring and if it stopped on a flower you would get warped to a bonus stage. There is another way to get one of these 5 bonus levels however and here's how...

Flip Cards Get a total of 700 points in World 1
Match Cards Get a total of 700 points in World 3
Roulette Get a total of 700 points in World 4
Scratch and Match Get a total of 700 points in World 2
Slot Machine Get a total of 700 points in World 5

Time Trial - Once you've beaten the game, you will be rewarded the Time Trial mode. To access Time Trial enter the File Menu like you would starting up a game. Your completed game file will have a star icon and will have the word "Clear!" under where the world should be. Click on your file and you will notice "Time Trial" on the bottom right corner where a question mark used to be. In Time Trial mode, you will be rewarded 5 extra levels, one for each world. You will also be able to play those levels with a black Yoshi.

Unlockables

Black Yoshi Play the secret levels in Time Trial
Time Trial Mode Complete the game once

Extra levels - To unlock the extra levels you will need to collect a 100 points on 8 stages (Including secret levels) of that world. Meaning you must get 20 red coins, 5 flowers and 30 stars to get 100 points. All together 800 points.

Extra 1 Get 800 points for world 1
Extra 2 Get 800 points for world 2
Extra 3 Get 800 points for world 3
Extra 4 Get 800 points for world 4
Extra 5 Get 800 points for world 5

New characters  -  You can get four new babies to ride on yoshi. They are baby peach, baby DK, baby Wario, and baby Bowser. Peach is unlocked at world 1-5, Dk at 2-1, Wario at3-5, and Bowser at 4-5. (note- Baby Wario and Baby Bowser will not remain with you forever, but peach and dk will.

Baby Bowser World 4-5
Baby DK World 2-1
Baby Peach World 1-5
Baby Wario World 3-5