This page in our guide for Assassin's Creed: Odyssey contains descriptions of all side quests that can be found on Southern Sporades - the archipelago consists of Kos and Samos islands.
A Business Opportunity
How to start: Free the prisoner in Artemisia Fort.
Description: Pick up the prisoner and take him out of the camp. Place the man on your horse and take back home.
Old Friends, Old Problems
How to start: Speak with Markos.
Description: Markos gives you wine - your goal is to deliver it to his old friend. Speak with the man. He and his guards will attack you. Kill them and then go back to Markos.
Farm in Flames
How to start: Speak with Markos after Old Friends, Old Problems quest.
Description: Markos has another mission for you. Your goal is to burn three silos. Go back to Markos to receive your reward.