Misko, the Great Bandit
Talk to the two male NPCs inside Dueling Peaks Stable, Domidak and Prissen. They'll shoo you off, but talk to them again and ask about "Misko". If you pay them 100 Rupees, Domidak will give you a clue about some sort of treasure stash.
"The little twin steps over the little river. My cave rests above that river's source."
If you can solve this riddle, you will find what is rumored to be a stash filled with rich treasure.
The "twins" mentioned in the riddle are the Big Twin Bridge and the smaller, unnamed-on-the-map Little Twin Bridge. Cross them both south of the Dueling Peaks Stable.
Then, go all the way past the end of the path towards the end of the river. You will see a waterfall. Climb up above the waterfall and you will see a cracked, bombable wall. Bomb it and you will see another bombable wall. Bomb that too and you will be greeted with a whole wealth of treasure chests.
There are also some treasure chests in the first area before the second bombable wall. You will receive a Flameblade and a variety of gemstones. Just opening one treasure chest will complete this sidequest but make sure you get them all!