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 grattitude.

The Stone Mask makes Link blend in to 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.