Riddles of Hyrule
You have to have begun the main quest "The Hero’s Sword". Learn more about it here.
Climb all the way to the top of the Great Deku Tree in Great Hyrule Forest. Then talk to Walton for a sidequest revolving around a series of riddles. You have to place the object in question on the leaf in front of where Walton is standing. Don’t worry, you can pick it back up. Walton won’t take it from you.
If you are having trouble finding the objects in question but you have them registered to your Hyrule Compedium, then you can use an upgraded Sheikah Sensor to look for them. Learn about how to upgrade your Sheikah Slate here.
For the first riddle, the answer is an Apple. Just a simple, plain, raw Apple.
The second riddle’s answer is a Fortified Pumpkin. You can buy them from the pumpkin patch in Kakariko Village mentioned in the riddle.
The third riddle is asking about a specific, warming mushroom. This is none other than the Sunshroom! You can sometimes find a Sunshroom behind the Hateno Ancient Tech Lab. They are otherwise scattered all over the warm and temperate parts of Hyrule. Sunshrooms are particularly common near the Gut Check Rock in the Eldin region.
The fourth riddle requires a Voltfin Trout. They are found in rivers and lakes all over Hyrule, most notably east of the Thundra Plateau and in the river leading up to Zora’s Domain.
The last riddle asks for a Lynel Hoof. Go defeat Lynels scattered all around Hyrule for one. The easiest Lynel is on Ploymous Mountain near Zora’s Domain, and a guaranteed Lynel is in the Coliseum Ruins after every blood moon.
After you have thoroughly impressed Walton, you will receive a Diamond for your effort.