At the 'Main' menu go to 'Extras' then select cheat codes. There you will see four boxes with pictures in them. Arrange the pictures in the correct order by scrolling through them then go to 'Enter Code' to activate the it.

Invincibility:
Baseball, Baseball, Baseball, Shirt

Unlock Big Head:
Hat, Helmet, Helmet, Hat

Unlock Big Feet:
Hat, Glove, Glove, Hat

Unlock Big Hair:
Baseball, Bat, Bat, Baseball

Unlock Sunglasses:
Glove, Glove, Helmet, Helmet

Unlock Underwear:
Hat, Hat, Shirt, Shirt

Unlock Paper Pants:
Bat, Bat, Hat, Hat


Card Set Unlockables
The following 6 Multiplayer maps can be unlocked by completing card sets. There are 20 sets in all.

Unlock Drone Dash:
Complete 2 Sets

Unlock Pig Paddle:
Complete 2 Sets

Unlock Walker War:
Complete 3 Sets

Unlock Saucer Smash:
Complete 3 Sets

Unlock Space Shooter:
Complete 3 Sets

Unlock Super Speedway:
Complete 5 Sets