Complete a level with 50 rings to have a big ring 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 the rings during the ending sequence to have Sonic show his fist pump pose from the Sonic The Hedgehog 2 and Sonic The Hedgehog 3 ending sequence, or turn into Super Sonic if you also have all the Chaos Emeralds.
On Casino Street Zone, run through the level and activate the "slot machines" to again extra lives. When you are satisfied with the amount of lives collected, pause game play and leave the level to save those lives. Restart the level and repeat the process as many times as desired.