While playing the game, press Start + Select + A + B.
Successfully complete the Leaf Forest level with Sonic to unlock Cream. Cream is extremely useful when facing Bosses because her Chao will attack it when you press B button. This makes it especially easy to defeat Bosses that are hard to hit, such as the fourth, sixth, and eighth.
Successfully complete the Music Plant level with Sonic to unlock Tails.
Successfully complete the Sky Canyon level with Sonic to unlock Knuckles.
Successfully complete the game with all seven Chaos Emeralds as Cream, Knuckles, Sonic, and Tails to unlock Amy.
Successfully complete the game with all seven Chaos Emeralds with any character to unlock Tiny Chao Garden.
Successfully complete the game with all seven Chaos Emeralds with any two characters to unlock sound test. Note: For more sounds, successfully complete the Extra Zone with Sonic.
Successfully complete the game with all seven Chaos Emeralds with any three characters to unlock the "Boss" option in time attack mode.
Successfully complete the game with all seven Chaos Emeralds with Amy, Cream, Knuckles, Sonic, and Tails to unlock a new level at the end of Sonic's map screen.
At the start of any non-Boss level, you can get an extra boost of speed out of the start gate. Quickly hold Right just before the counter says "START!". This may require a few attempts to get the correct timing. Also, use the following for a speed boost on Sky Canon. Choose any character that is fast. Select the Sky Canon Act 2 stage. Once you find a fan that blows you to get more speed, go back near the fan. Wait a few seconds and their speed will increase. After this process, turn around. This is useful with Tails because he starts by walking, then jogging, and finally flying.
When you get to a red ramp in the level 1, you can get farther and higher. When your about to jump on it, press A when about to jump. This only can be done on level 1.
When playing as Cream the Rabbit, you can use Cheese (her Chao) for more than attacking enemies. You can also call Cheese to snag Special Rings.
On the huge machine at the end of the final level, use Tails' propellers to hit his head. Note: You need to hit him with the propellers precisely.
This glitch works for Knuckles, Tails, and Cream. Go to a Boss with one of those characters and fly or glide to the left side of the screen. They will be blurry and you will see doubles of them. Also, if you are Cream and she gets tired while flying, when Cheese tries to pull her up there will be two Cheeses.