Unlock Metal Sonic
     To  unlock  Metal  Sonic  you  must  beat  the  game  with Sonic,
Knuckles, Silver, and Shadow. (In no particular order).

Unlock new cups
     To  unlock  new  cups (as well as obtain Chaos Emerald cards) for
Cup  Circuit  mode,  you  have  to  complete  Easy,  Medium,  and Hard
challenges in specific zones in Challenge Mode as follows:

Blue Chaos Cup:    Complete all challenges for Death Yard Zone
Cyan Chaos Cup:    Complete all challenges for Colosseum Highway Zone
Green Chaos Cup:   Complete all challenges for Forest Falls Zone
Grey Chaos Cup:    Complete every challenge in the game
Purple Chaos Cup:  Complete all challenges for Meteor Base Zone
Red Chaos Cup:     Complete all challenges for Sky Park Zone
Yellow Chaos Cup:  Complete all challenges for Crystal Mountain Zone

Unlock Skins
     In  order  to  unlock  certain skins you must complete story mode
with certain characters:

Knuckles Skin:     Complete story mode with Knuckles.
Metal Sonic Skin:  Unlock Metal Sonic.
Shadow Skin:       Complete story mode with Shadow.
Silver Skin:       Complete story mode with Silver.
Sonic Skin:        Complete story mode with Sonic.