So I beat the game for the second time today and my second playthrough I decided to go with the nonlethal style. I completed every side mission for slackjaw and the old lady, I found every rune, almost every bone charm, and did it all in 14 hours without getting caught or killing anyone. Now I don't know what to do except hunt down achievements (Xbox 360). So my question to you is, what other things are there to do?

To teach me what arcane black magic you use to be that fast.

This. It's literally impossible to do that in that ammount of time. Well, maybe if you play on anything but HARD or VERY HARD and don't read/listen to anything or explore beyond the objective markers and hearts rune/bone charm markers.

Did you get both high and low chaos endings? Have you tried out every power? Did you find all of the Sokolov paintings? Try playing through without upgrading any powers it is a lot of fun and you get an achievement for doing it as well.

There's a list of thing's you can still do but it depends how creative you are to do them.