Metal Sheen:
Hold A and Y after selecting a 2-player level to have a "metallic" skin covering over your team.

Unlockable: Last Story:
Collect all 7 Chaos Emeralds and finish all stories

Unlockable: Super Hard Difficulty:
Beat the game with 141 Emblems and all A ratings.

Unlockables: CG Theatres:
Team Chaotix Cinema: Complete Team Chaotix in Story Mode
Team Dark Cinema: Complete Team Dark in Story Mode
Team Rose Cinema: Complete Team Rose in Story Mode
Team Sonic Cinema: Complete Team Sonic in Story Mode

Unlock New 2P Modes:
2P Action Race Unlocked from start
2P Bobsled Race Collect 80 emblems
2P Expert Race Collect 120 emblems
2P Quick Race Collect 100 emblems
2P Ring Race Collect 60 emblems
2P Special Stage Collect 40 emblems
2P Team Battle Collect 20 emblems