Super Mario World 2: Yoshi's Island (SNES)
In a level that you've already finished, press Start to pause and then press Select to go back to the Map Screen.
Practice the mini-games
On the map screen, point to a stage with the cursor, then hold Select and press X, X, Y, B, A. If entered correctly, a new menu will appear with several mini-games to choose from.
Defeat Naval Piranha with one egg
This is useful if you want to ensure taking zero damage from Naval Piranha, or if you just want to beat the level quickly. In World 3-8, when you're swimming towards the boss battle, you'll see a long, skinny platform made out of circles, with a small Piranha Plant at the other end. At this point, if you go too far to the right, Kamek will appear and enlarge the Piranha Plant, which is not what we want. Stand on the first circle of the platform (anything past that will trigger the boss battle sequence), and hit the small plant with an egg. Kamek will fly in and say "OH, MY!!!", and then the plant will explode.
When you start World 5-4, immediately enter the door on your left. In that room, kill off the Chain Chomp with a Winged Cloud Maker from your item inventory (Pause screen). Enter the secret door that appears, and hit the Message Block to learn the mini-game code listed above.
Unlimited 1-Ups: World 1-7
In the beginning of World 1-7, have Yoshi eat a Melon Bug (the things that roll up into a ball). Don't swallow it, but keep it in his mouth until you come to a pipe that produces Shy Guys. Spit out the Melon Bug and have it stop close to the pipe. Stand somewhere so the Melon Bug is between you and the pipe. As the Shy Guys pop out of the pipe they will be killed by the Melon Bug. After killing eight Shy Guys, every additional Shy Guy killed is a 1-Up.
Unlimited 1-Ups: World 4-1
Play through World 4-1 until you come across two long pipes hanging upside down. Kill the Piranha Plants, and then swallow Shy Guys until you have six eggs. Grab a Koopa Troopa, stand on the pipe which previously held the lower Pirahna Plant, and then spit out the shell between the two lower pipes. As it bounces back and forth, shoot off one egg. Shy Guys will begin pouring out of the other pipe and will be killed by the shell. After killing eight Shy Guys, every additional Shy Guy killed is a 1-Up.
In World 6-7, you'll find the Key inside a big mushroom-shaped structure made up of the breakaway wall stuff. And in World 6-Extra, right after the first red elevator lift is a group of five lifts that spell out YOSHI.