Costumes
God of War 2 comes with a large set of unlockable costumes. They can help you out, but some have disadvantages.
Beat the game on any difficulty to unlock the Hydra armor and the Cod of War costume. When Kratos wears the Hydra armor, the value of his armor and any green, gold or blue orbs he collects is doubled, but his strength is cut in half. The Cod of War armor is visually ridiculous but doubles the value of all orbs.
For the Dark Odyssey costume, complete the game on "God Mode." While wearing this costume, Kratos' defense is cut in half, but his strength is doubled. Red orbs also increase in value.
Unlock the Athena and Hercules costumes by defeating the game on "Titan" mode. With the Athena costume, Kratos' defense will drop slightly, but the value of red orbs will double while the green and blue orbs increase significantly. When wearing the Hercules costume, Kratos' armor works only half as well, but his overall strength doubles.
Collect 20 Cyclops Eyes to unlock the General Kratos costume. This costume has several drawbacks. Your strength and armor will be cut in half while the value of any green, gold or blue orbs you collect is lowered. The only advantage is a dramatic increase in red orb values.
Finally, the God of War Armor is awarded to players who beat Challenge of the Titans with a "God" ranking or higher. There are no drawbacks to this armor, and all of Kratos' stats double.
Unlockables
To unlock the Challenge of the Titans, you'll need to complete the entire game. Keep in mind that Challenge of the Titans requires players to work their way through seven different rooms to defeat it. Receive the "Titan" rank while playing the game on Challenge Mode and you'll unlock the Arena of the Fates where you can duel it out with various enemies. If you're curious about unlocking "Titan Mode," a harder difficulty to play on, all you have to do is defeat the game the first time and it becomes available.
Secrets
These four items are found hidden throughout the game and allow you to collect various rewards. According to IGN.com, here's what to keep your eye out for during the game. Gorgon eyes and Phoenix feathers are both hidden inside chests. For every six Gorgon eyes you collect, you'll be rewarded with a larger health bar. Collect six Phoenix feathers to lengthen Kratos' magic meter. Four urns are hidden on their own throughout the game. The last two urns must be earned by beating Challenge of the Titans with the rank of "Mortal" and again with the rank of "Spartan."