As the question states, what do you always bring to a dive? Not necessarily down under the surface, but say in the gear bag? Except the obvious such as gear, logbook, certifications and so on.
For example, I don't go diving without having a "save-a-dive" kit with me, containing extra mask strap, o-rings and some other stuff that can help when something breaks.
Interesting to hear what other people consider necessities when going diving.