Cheat mode - At the main menu, press Z, R, L, B, X, Y. If you entered the code correctly, a voice will say "Go cheese". Start a new game to begin with the Soul Reaver and Iron Armor.
Easy health - Immediately after clearing a checkpoint, if you are low on health, reload the game. You will be back exactly where you just were, except your health meter is now completely full.
Easy kill - After defeating Sebastian and acquiring the Dark Gift Charm, use it on a enemy that you cannot charm. It will appear as if they have a headache. Quickly run towards the enemy and attack. Repeat this until the enemy is killed.