To get Knuckles' leather shoes, get the Buzzer 2002, Coconut 2002, and Turtloids 2002 cards.
Metal Sonic
Successfully complete the game with Sonic, Knuckles, Silver, and Shadow to unlock Metal Sonic. Alternately, collect the "Metal Madness", "Metal Overlord", and "Neo Metal Sonic" cards.
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:
Effect - Code
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
Silver's leather shoes
To get Silver's leather shoes, get the Chaos 2 (1998), Chaos 4 (1998), and Chaos 6 (1998) cards.
Skins
Successfully complete the indicated task to unlock the corresponding character skin:
Knuckles: Successfully complete Story mode with Knuckles.
Metal Sonic: Successfully complete Story mode with Metal Sonic.
Shadow: Successfully complete Story mode with Shadow.
Silver: Successfully complete Story mode with Silver.
Sonic: Successfully complete Story mode with Sonic.
Sonic's ice suit
To get Sonic's ice suit, get the Miles "Tails" Power 2002, 2004, and 2005 cards.
Sonic's leather costume
To unlock Sonic's leather suit, get the Sonic The Hedgehog, Sonic The Hedgehog 2, and Sonic The Hedgehog 3 cards. To unlock the gloves, get the Cream The Rabbit, Cheese, and Cream and Cheese cards. To unlock the shoes, get the Sonic and Amy, Amy Rose 2004, and Amy Rose 2005 cards.
Knuckles' leather shoes...