I carry spare inner tubes (2), tyre boot, brake/gear cables, chain masterlink, 3 or 4 spokes. On longer tours I carry a spare set of brake blocks (because modern blocks need cutting to fit my old bike).
A few spare M5 bolts, zip ties, ductape, wire should get you home through most other failures. I cant think of any other spare that you would carry unless you are crossing a vast wilderness on your own.