Successfully complete a level with 50 rings, and a giant ring will appear behind the end sign. Jump into the giant ring to advance to a bonus level. A Chaos Emerald will be at the end of all bonus levels.
Collect all seven Chaos Emeralds. Then, enter any level, and collect 50 rings to be able to transform into Super Sonic.
Collect all seven Chaos Emeralds in the Special Stages to unlock an alternate ending and Episode 2 preview.
During the ending sequence, collect all the rings, and Sonic will do his fist pump pose from Sonic The Hedgehog 2 and Sonic The Hedgehog 3 ending sequence, or turn into Super Sonic if you also have all seven Chaos Emeralds.
Play the Casino Street Zone. Run through the level, and activate the slot machines to get extra lives. Once you get the desired number of lives, pause the game, and leave the level to save those lives. When you want more lives, restart the level, and repeat the process as many times as desired.
Get 50 coins, and finish an Act. Jump into the giant ring immediately after the Dr. Eggman sign. Try to get the emerald. If you are about to run out of time or hit the exclamation circle, pause the game, and retry. You will only restart the Emerald Zone. You can do this as many times as desired.
Enter a slot machine as Super Sonic to always get a jackpot.