Campaign Mode Select:
At the player selection screen press C, C, C, C, C.

Level Edit:
At the main menu hold Ctrl and press E, E, E, E, E, E.

Abort Losing Round:
Immediately press F10 if your character is destroyed to keep the round from being counted out.

Bonus Graphics:
Enter the "Data/Sound" directory of the game to find hidden graphics of the game's programmers.