Some things that do not go in my camera bag but are essential for a photography outing. A multi tool to help tighten, loosen or generally pursued things. Maybe not even thing for the taking of pictures. Perhaps something in the car or at the scene when I arrive, or, leave.
Spare batteries, for the camera, but also for the other gadgets that are essential for a good outing. Batteries for camera remote releases, for recording audio on videos etc Battery tester, to make sure the spare batteries you are taking have a good charge and will do the job. A selection fo micro screw drivers drivers or driver with multiple screw heads for difficult and awkward things such as battery covers. Instruction manuals. For new gadgets, or gadgets where you are using new features. Either have them as PDFs on your phone or, take paper ones with you.