Armor puzzle in Lava stage
Just before you get the mini-gun, there is a door on your right. Open it and there will be armor on the other side. You must to step on the correct letters; the order is: S, E, R, I, O, U, S.
Cheat Mode
At the main menu, hold R and press Black, White, Black, White, Black, White repeatedly until the cheat option appears at the bottom. Then, press X when at the menu to access the cheat menu. The cheat options can also be activated in online games, but may require the game host to first enable cheat mode.
Defeat Zum-Zum
In the level when you must defeat Zum-Zum, you must first destroy all of the giant sunflowers so that Zum-Zum cannot regain its life. Once you destroy the sunflowers, you can attack Zum-Zum and kill it before it recharges.
Duke Nukem Forever reference
There are several passing remarks in the game about a "Blonde guy taking forever". This a reference to the as-yet-to-be-published PC game Duke Nukem Forever.
Armor puzzle in Lava stage...