Unlock Bonus Level
To unlock a bonus level that becomes available on Sonic's map, beat the game and collect all Chaos Emeralds with Amy, Cream, Sonic, Tails and Knuckles.

Unlock Boss Option in Time Attack
To unlock the Boss option (Time Attack Mode), beat the game and collect all Chaos Emeralds with three characters.

Tiny Chao Garden
To get the Tiny Chao Garden, collect all 7 Chaos Emeralds and beat the game with Sonic.

Unlock the characters
To unlock the unavailable characters, beat the levels with Sonic:
Beat Leaf Forest to unlock Cream the Rabbit
Beat Music Plant to unlock Miles "Tails" Prowler
Beat Sky Canyon to unlock Knuckles the Echidna

Saturn Sonic Spin
In either level, if you choose Sonic , and if you have blue or green barrier, and you bounced off a spring, press Forword or Backwards + R, it will make you look like Saturn