The following codes unlock the extra stadiums on the Stadium Select screen
golden - Bayside Park
roma - Coliseum
catapult - CVN-99
cactus - Desert
Easter Egg Island - idol
idol - Emerald Stadium
alien - Hangar Stadium
stucco - High Bluff Stadium
gridlock - I 989
lunar - Space Station RDZN
sandy - Spring Breaker
serpen - Temple of Apep