The Hero's Cache
Kass, the accordion-playing Rito who travels all over Hyrule, is standing out on the tallest rock column in Kitano Bay near Hateno Village. It's easiest to reach by Paragliding from the nearby Ancient Tech Lab.
Speak with Kass to hear a song about a treasure chest left behind by an ancient hero. The "17 of 24" part of the verse refers to the time of day the treasure chest can be found, which is 5:00 PM. If you wait until then, you can use Magnesis to pull up the treasure chest nearby.
Paraglide directly east in the direction Kass is facing as he plays the accordion. Scope out the area with Magnesis and you will see the treasure chest. Pop it open for a shiny gold Rupee worth 300 Rupees!