Extra Levels...
	Hard Mode...
	Music Player...
	Red star rings...
	Super Sonic...

Collect all of the red rings in a zone and you will unlock an extra level in that zone.

Clear the game once to unlock harder versions of the main levels.

Beat the final boss.

Level	How to unlock
Desert Ruins Zone Extra Level	Collect all 15 red star rings in Desert Ruins.
Frozen Factory Zone Extra Level	Collect all 15 red star rings in Frozen Factory.
Lava Mountain Zone Extra Level	Collect all 120 red star rings in the game.
Silent Forest Zone Extra Level	Collect all 15 red star rings in Silent Forest.
Sky Road Zone Extra Level	Collect all 15 red star rings in Sky Road.
Tropical Coast Zone Extra Level	Collect all 15 red star rings in Tropical Zone.
Windy Hill Zone Extra Level	Collect all 15 red star rings in Windy Hill.

Clear all of the special stages to collect the Chaos Emeralds and you'll gain access to Super Sonic.