Beat the God of War Challenges to get the extra costumes which have the following attributes:
(1) Dairy Bastard:
(2) Chef of War:
(5) Ares Armor:
Infinite Experience Source:
In Atlas' Quest in Pandora's Temple kill every armored guards who try to knock you off the series of stone walls, and get to the tallest ledge. Once there, a screaming harpy will appear and Blood Orbs will start to attract towards you, which will last for as long as you are in the area.
You get the first one by beating the game in Spartan Mode. For the second, use Lance of the Furies ( L1 + Circle ) to destroy the Ares and Minotaur statues at Olympus. You may also beat the God of War Challenges, kill Ares, then stab the red carpet which leads to his throne room. After you have stabbed the carpet enough times, the second Secret Message will be available.
Get Both Muse Keys:
The first key is through a breakable wall in the underwater passage when you have opened Poseidon's door. The second key is at the top of the ledges you have to climb during The Challenge of Poseidon. When you are at the top, take a right for the chest. If you missed either of those, there is another to be had behind the Atlas statue, when you cock it into throwing position. Then go to the door in The Rings of Pandora to use the keys.
After beating the Hydra and you are back on your ship, jump on the bed and press R2.
When you are in your ship at the beginning of Act 2, go to the two women on your bed, jump on it, and press Circle.
Thanks to Revolution readers Phoenix, f1re, Jacob, Fisto, Monster_Zero, savior, and Joe Diaz!