Animorphs. Loved those books as a kid and revisited them as an adult, and forgot how dark they got. Child soldiers, war crimes, the horrors of losing yourself to the hive mind or being forced into the body of a small animal (not talking about Tobias here).
Plus the books are also about friendship, which many youths want to see more of than romance.
My girlfriend's wisdom teeth, along with two of mine.
I used to collect animal bones in my hometown, where hunters carelessly tossed what they didn't want behind nearby train tracks. Found a whole boar skin once, but only took home little knuckle bones. I might still have them somewhere.
I have a deer skull on my shelf, but I think most people would overlook that. Human teeth left in storage among other junk? Not so much.
I didn't even know that was an option, but don't want to get my expectations up.