How are the side quests done?
Not unlike any other Soul, on which you interact with an NPC under certain circumstances, then you have to interact with him or her again on another place enduring a different circumstance. I was tasked, for example, to find the missing partner of a knight, but I was never able to track him down, thus failing the sidequest altogether.
I don't agree with this one. There are quite a few optional areas: Smouldering Lake, Archdragon Peak, Consumed Kings Garden, and Untended Graves.
I've just recently found the first one, so I kinda regret that comment now; that being said, the thing I had to do to enter it was something I think I'd never had found by myself had not another user told me about it. The only one I found by myself (so far) was the last one you mentioned it, and that's basically a Firelink Shrine reskin. I'm not even counting Consumed King Garden because it doesn't really feel like a sideplace of the magnitude of a secret area (heck, the game doesn't even award you a trophy because it probably didn't consider it secret enough), and the only thing it takes you to is to Untended Graves, so...
Maybe I'm being a bit harsh here, but that's how I feel. No clue about Archdragon Peak though.