Take Back the Sea
Go to Lurelin Village for this sidequest and talk to Sebasto. He will ask you to do a favor: Elimiate the monsters at Aris Beach to make the fishing grounds safe again.
Travel to Aris Beach and you will see a huge enemy encampment built out into the sea. The easiest way to minimize actual fighting is to use Cryonis or a raft to approach them and then snipe down the Bokoblin archers on the lookout posts. Then you can climb up to those posts using Revali's Gale if you have it.
From the posts you can shoot down as many of the Bokoblins and Moblins as you can from afar. Then Paraglide to the encampment itself and take out the rest. Be sure to open up the treasure chest for a Royal Halberd.
Return to Sebasto and he will give you a silver Rupee worth 100 Rupees for your efforts.