These are the codes for unlocking the extra stadiums on the stadium select screen:

Stadium - Code

Temple of Apep - serpent
Bayside Park - golden
Coliseum - roma
CVN-99 - catapult
Desert - cactus
Easter Egg Island - idol
Emerald Stadium - needle
Hangar Stadium - alien
High Bluff Stadium - stucco
I 989 - gridlock
Space Station RDZN - lunar
Spring Breaker - sandy