Optional Mission: Find the Cipher / Code Book - Soldier's Field: Bioshock Infinite

Quite far on in the Soilder&Field level in the Hall of Heroes Plaza (after you try to summon the airship), you&find this Vox Populi code on a wall in the back of the restaurant. Simply cross the square and enter the other room, where you&find a cannon right in front of you. Grab the code book from the cannon, then head back to the wall, and wait a second for Elizabeth to do her thing. Once she&cracked the code, she&say you need to tip your hat, so head out to the left, and look for a hat in the next room. Activate it, to reveal a secret room with a delicious RPG.