The Legend of Zelda: Majora's Mask/Stone Cold Soldier

Requirements

 * Lens of Truth
 * Red or Blue Potion

Rewards

 * Stone Mask

What To Do
In the path that leads to Ikana Canyon, find a circle of rocks. Use the Lens of Truth to find Shiro, a soldier who has been there for a long time, trying to get others to notice him, but nobody ever does. He will ask you for medicine to make him feel better. Give him a life-restoring potion (Red or Blue) and he'll give you the Stone Mask as a symbol of his gratitude.

The Stone Mask makes Link blend into his surroundings to hide him from enemies. This can be most useful in the Pirate's Fortress, so get this mask before going to the Great Bay.

In The 3DS Version
In the 3DS version, you must traverse the Gerudo Fortress to make your way to Shiro. He is in the middle of a large courtyard in the fortress. To get to him, hide behind boxes while knocking out the guards so they don't spot you. Once you manage to get past the guards without them spotting you, talk to him, give him the medicine, and he will give you the Stone Mask. Then you can walk right in front of the Gerudo Guards if you want (after you equip it, of course). They will not react to you.