Find the Fairy Fountain
This sidequest, if you choose to do it before beating the game, will provide a huge boon because fairy fountains in Hyrule give you the ability to upgrade your armor!
Speak with Pikango. He is an artist who hails from Kakariko Village. He will mention wanting to find a fairy fountain. If you tell him you're interested, he will tell you to follow him.
Unfortunately, just past Ta'loh Naeg Shrine, Pikango will get too tired to go on. He will ask you to find the fairy fountain in his stead and prove that you found it with a photo on your Sheikah Slate.
Continue forward along the path to where you see a lot of glowing orange mushrooms and what looks like a large, closed flower bulb. If you press [A] once you see the prompt to examine this flower, you will hear a voice talk about needing rupees. It sounds like a total scam, but trust me on this one. Pay up if you can and meet Great Fairy Cotera.
Note: There are four fairy fountains scattered across Hyrule. The cost to unlock each depends on how many other fountains you have unlocked. If this is your first fountain, it will cost you 100 rupees. The second fountain costs 500, the third 1,000, and the final fountain costs a whopping 10,000 rupees!
Her sense of style and immense size aside, Cotera will be able to enhance your armor. Armor upgrade can do all sorts of things, such as boost defense, boost any side effects it gives, and if a whole upgraded set is worn, give you other special bonuses.
Upgrade if you wish, then take a picture of the newly opened fountain and take it back to Pikango. As a reward, if you haven't completed Captured Memories yet, he will give you more specific hints on where each memory is located.