At the world map, press R.
Successfully complete all 96 goals to get another color scheme for the world map, alternate enemies, a new introduction sequence featuring Yoshi, and other changes. You will also be able to start at any location on the map. Press Select at the main screen, then select a level at the status screen. Press A to start at that level instead of moving to it on the map.
Note: This can only be seen if you have completed the entire game. Turn on the Game Boy Advance, allow it to idle and watch the opening sequence. After the part where Mario/Luigi have flown with capes over the dolphins, the screen will go blank. It will then show a new scene where Peach rides a green Yoshi with a yellow one following.
Successfully complete the game and go to Yoshi's House or go to Yoshi's House with Yoshi.
Use the following trick to get yellow, blue, red Yoshis in levels other than Star World. Get to the Star World and get one of the Yoshis. You will now be able to get those types of Yoshis in other worlds, and they are fully grown. Note: This does not always work and sometimes you will still get a green Yoshi. After unlocking the red, blue, and yellow Yoshi (feed young colored Yoshi five enemies), you can pick the color you want. If you are Fire Mario/Luigi, you will get a red Yoshi. If you are Cape Mario/Luigi, you will get a blue Yoshi. If you are Big or Small Mario/Luigi, you will get a random color Yoshi (usually either yellow or green). This trick works well with the Top Secret Area.
In the Star Road levels, grab and hold a baby yoshi then immediately feed it a Star (the item that makes Mario invincible for a brief period of time). It will instantly grow into a big Yoshi. This is especially useful when you need to act fast and you do not have time to feed it six enemies.
When Yoshi eats a multicolored shell, he will be able to fly for a short time.
Remain idle at the "Choose a game" screen and watch Yoshi. After awhile he will start running then start walking.
Enter any of the special levels and remain on the world map. Leave the game idle until the music changes.
Collect all five Yoshi Coins from each level. An animation will appear showing them change from Yoshi Coins to Princess Coins.
After you defeat Bowser, all of the castles and fortresses you defeated will be reopened so you can go back to them. Note: This was done by defeating Bowser before completing all seven castles.
Once you complete the special stages, you will be able to save your best scores on them. The bottom of the screen appear like: BEST (score) M or L (which character set the best score) (current score).
For every million points, one star will appear on the "Choose a game" screen on top of the type of game.