Goron Sidequest

From Zelda Wiki.org

The Goron Sidequest is the lengthiest non-required game plot detour in The Minish Cap, that incorporates Kinstone Fusion and luck to achieve one of the four Empty Bottles of the game.

Located at the left-hand side of the Lon Lon Ranch region, there resides a Goron trying to break down a rock wall. By fusing a kinstone with a certain Minish in the game, initiating the goron sidequest, the Goron will bust through the wall and form a cave.

Over the course of the game, different kinstone fusions with Mysterious Walls and one with the farmer Eenie, who resides in the Eastern Hills region, will trigger multiple Gorons to aid the original Goron in breaking down several impenetrable walls within the cave until six Gorons have been rallied. At that point, the Gorons bust the final wall, and a chest containing an Empty Bottle will be waiting for Link at the end of the room.

Mirror Shield

By fusing yet another kinstone with the final Goron to the right in the cavern, a Biggoron will appear over Veil Falls, whom will eat your shield after Vaati has been defeated, only to return it to you later as a Mirror Shield. The Mirror Shield, like the Light Arrows, Remote Bombs, and extra Empty Bottles are not required to complete the game, but add a sense of completion to the game once it has been finished.