The Priceless Maracas

"The Priceless Maracas" is an important side quest you can do pretty early on in the game.

Game progress required: Clear the Great Plateau, have at least 1 Korok seed

How to start the quest: On your way north to Kakariko Village, or south from Kakariko, you should run into a strange-looking character named Hestu. Talk to him, and he will tell you that his maracas have been stolen by some monsters nearby.

Guide:

Just up the path from where Hestu stands, a bit to the right, is a camp of Bokoblins. Simply defeat them in whatever way you like, and the skull chest on top of the lookout post will change color and unlock. Retrieve Hestu's maracas from the chest and bring them back to him. He will realize then that the other Koroks have taken away the Korok seeds he uses in his maracas. Offer him one of your Korok seeds, and he will expand one of your inventory slots (you can pick from your weapon, bow, or shield stashes) and the quest will be complete.

Reward: Once you bring Hestu his maracas, he will now be able to expand your inventory in exchange for Korok seeds.