The adventure that is Witcher 3 is neatly divided up into several basic plots and a while lot of sub-plots in the form of Side-Quests, Treasure Hunts, Scavenger Hunts, and Witcher Contracts.
Each region has its own set of these quests - and each of the quest types follows a basic formula. In the case of Side Quests, that formula ranges from simple fetch-quests to some fairly complex mini-stories in their own right.
The following sections are divided up by region, with the Side Quests that appear in each appearing... In each...
Since the order that you are likely to encounter them is unlikely to be the order in which WE did, what we have done is arrange them alphabetically, so that you have an easier time accessing the one you want, and you can dip in and out as required.
You may find that observing the published Suggested Level is a good idea...
Side Quest Strategy: As there is no upper limit to the number of quests you can flag what we recommend you do is flag ANY that you CAN.
That way you have a selection of level-appropriate side quests for you to do in the interest of gaining levels via XP as well as Wealth and experience.
You can find the guides for each side quest in the appropriate sections below, Good luck!