Use the debug mode cheat. Start any level that has a special stage ring. Next turn into your favorite object. As an object go to the center of a ring. Now hit B twice quickly. When you get out of the special stage you will look like that object for the rest of the level.
Enter the Level Select code, then press A + START to choose your starting level. Once the action starts, press C to create an item, B to toggle between items, and A to select items.
Different Battle Music
Just before Before you fight Robotnik in the Hydrocity Zone, act 2, the boss music will come. Keep Sonic's head under the shallow water until he is about to drown, then pop out of the water. You will hear the boss music from Sonic and Knuckles.
To gain all the chaos emeralds, enter the Level Select code, then highlight "Special Stage" and press C + Start. After receiving the emerald, press A + Start and repeat the process until you have them all.
Alternatively, enter the Sound Test code and play sounds 2, 4, 5, 6 in order. If you did it correctly, the Emerald sound will play.
To do this trick you must get to the first level of the Launch Base Zone. Set off the first alarm, stop Sonic in the middle of it, and do a Spin-Dash attack. The flybots will continue to attack, and eventually their value will increase to 10,000 points! Every five flybots will gain you a life and, if you're patient enough, you can earn up to 99 lives!
Extra Rings in Debug Mode
Enter Debug Mode then select a stage. Next press B, A (you must have Sonic and Tails on-screen.) When your cursor changes into a ring, paste it on Tails. Repeat this process and you can gain up to 900 rings and several 1-ups.
This code requires a Sonic 2 cartridge. Insert Sonic 2 and enter the Sonic 2 Level Select (play sounds 19, 65, 09, and 17). Return to the title screen, hold A and press Start. To transfer this to Sonic 3, hold A and carefully remove the cartridge without turning off the Genesis. Insert Sonic 3 and press Reset. Finally, release A and press Down twice at the main menu.
Note: You might notice a few levels you haven't seen before, but you can't select them. Those levels were originally intended for the game, but were pulled in order to release the game on time.
Mecha Sonic and Tails
Use the Debug Mode code in Launch Base act 2. Beat the game. When Sonic or Tails jumps towards you, activate Debug Mode and get out of it immediately. You will be able to use Sonic or Tails in the background for a short time, but they will look like robots.
Play at Night
Enter the Level Select code, then press C + Start to choose your starting level.
Random Starting Location
Enter the sound test menu choose a level. Look for a star post, touch it and hold A and press START. Then choose another level. If done correctly the title of the zone will change. Beware you might start out in a wall so use debug mode to fly out.
Secret Special Stage
For a bonus stage, enable Level Select, then play tune 1F. Finally, highlight stage 2 and hold A while pressing Start.
First enable Level Select. While playing, you can pause the game and press A to restart, B for slow motion, and C for frame advance.
Enter the Debug Mode code, then press UP at the main menu to reveal the Sound Test option.
To become Super Sonic without getting all the Chaos Emeralds, first enable the level select and debug mode. When the game begins, create a power-up box. There should be an 'S' on the box. Jump on it to grab 50 rings and become Super Sonic!
Enable Level Select, then press Down to make sure you have the sound test (don't select it yet). Start a one-player game and choose Tails in the Data Select screen. When the timer appears, press RESET.
Enter the Sound Test menu and choose any zone. Hold A and press Start. You should be Tails and be in debug mode. From here press B, then press A until you see a box. Press C to select it, then press B to return to Tails.
To enable Super Tails, jump on the box. Tails will look weird and can not be hurt or lose any rings. He can still get squashed and drowned, though, so be careful in the Ice Cap zone. You will have do this at the beginning of each zone. Robotnik can hurt him at the very end of Launch Base zone-act 2.
Unlimited Bonus Stages
Enter the Level Select code, then highlight the "2P Vs" option and press START. You will enter a series of bonus stages that last until you reset the system or press A+START.
Enter Debug Mode and press B + C. You'll be treated to a view of Sonic and Tail's moves.