Secrets (StH2Retro)

From Sonic - Wiki on Neoseeker

[edit] Play as Knuckles

Buy a copy of the Sonic & Knuckles lock-on cartridge (if you haven't got one already) and put the Sonic 2 cartridge on top of it. When you switch the power on, you'll be able to play through the game as Knuckles, with all the abilities he has in Sonic & Knuckles.

[edit] Super Sonic

After you get all 7 Chaos Emeralds, collect 50 rings then jump into the air in any zone and you'll turn into Super Sonic! Super Sonic is gold, runs at a much faster speed and is invincible. Note: You can only get this while playing as Sonic.