The game is of linear type, thus in order to complete all side quests you should simply talk to all NPCs about everything, ask about details and always agree to help them. While traveling across the levels all barrels and crates should be destroyed, and secrets discovered. It's worth paying attention to the fact that the following [SIDE QUESTS]:
- Work of Westernesse (chapter 1);
- Elf-stones (chapter 3);
- A Gem No Mine Can Yield (chapter 5);
... are about finding specific items hidden in barrels or crates. Unfortunately, they are generated at random and there's a chance that even after completing, for example, chapter 1 several times you won't find all Westernesse Artifacts necessary to do this quest.
Completing 15 (main or side) quests will result in unlocking the [ACHIEVEMENT] Many deeds, great and small. When you complete the [SIDE QUEST] The Captive you will unlock the [ACHIEVEMENT] Friend of the Woodland Realm, while completing the [SIDE QUEST] Cult of the Lidless Eye will result in unlocking the [ACHIEVEMENT] The Lidless Eye.
Below you can see a list of all side quests as well as chapters where you can begin a particular side quest:
A Gem No Mine Can Yield
Cult of the Lidless Eye
Forewarned is Forearmed
The Last Sons of Cardolan
The Seer's Words
Chapter 3 / 4
Work of Westernesse