Natures
Lax raises Defense by 10% and lowers Sp. Def by 10%. It is applied at the very end of the formula, after IVs and EVs, so the higher the stat, the more the nature gives and takes.
Raises Defense, lowers Sp. Def — a wall that picks a side. Rarely worth it, because it trades one kind of bulk for the other.
Flavor
Likes Sour, dislikes Bitter. This matters for Poffins, Pokéblocks and Berries: a Pokémon eats a disliked flavor reluctantly, and healing works worse.