Enter the following codes while holding L and R at the Press Start screen:

Infinite health:
Press Up, Right, Up, Left, Down, Left, Down, Right.

Endless combo meter:
Press Left, Right, Down, Up, UP, Down, Right, Left.

Every upgrade:
Press Down, Up, Down, Left, Down, Right, Up, Down.

Every bonus:
Press Left, Down, Left, Right, Left, Left, Down, Right.

Dark Knight difficulty setting:
Press Right, Up, Up, Right, Left, Down, Right, Up.

Thanks to Revolution readers Pee Kin and Lucky Blue!