Debug Mode
Enter the level select code, then press A + START and choose your level. Once the action starts, press C to create an item, B to toggle between items, and A to select items.
Extra Rings
(Note: Be sure to have both Sonic AND Tails on-screen) Enter debug mode, turn into the ring sprite. Next, just keep pasting rings onto Tails.
The Real Supersonic
Go to stage four and collect 50 or more rings. Press A, B, and C at the same time. The music will change. You will be a golden Yellow and twice as fast. You can jump twice as high too. I would collect 70 rings if I were you. Once you use this code, every 1 or 2 seconds makes you lose a ring. When you lose all of your rings or beat the boss to go to Act 2, you will become normal again.
View Animations
Enter debug mode and press B+C. Then Sonic and Tails will show-off their moves!