Cheat mode
Select the "Enter Code" option at the "Bonus Content" menu. Then, enter one of the following codes at the "Secret Cheat" screen to activate the corresponding cheat function. After a code has been accepted, you must then turn on the unlocked cheat at the "Secrets" menu. 

Invincibility
Enter Kevin, Big Chill, Swampfire, Kevin as a code. 

All combos
Enter Swampfire, Gwen, Kevin, Ben as a code. 

Infinite Aliens
Enter Ben, Swampfire, Gwen, Big Chill as a code. 

Level Lord
Enter Gwen, Kevin, Big Chill, Gwen as a code. 

Goop
Enter Ben, Swampfire, Kevin, Big Chill as a code.