As a lot of the other divers have already given good answers, I'll just try to supplement with other accessories that I think can be important as well.
Wrist slates/Wetnotes - Found this invaluable tool if you are looking to improve your diving skills. For me, I record the objectives and skills that I want to practice on the slate, and also my performance on those skills after I perform them. I also take notes on my trim, buoyancy, breathing, stress management, situational awareness, problems that manifested during the dives etc. These information goes into my dive log, and help improves diving skills very fast if you take the time to briefly read through the logs each time before you go diving.
Action camera - can be used to film self or buddy, and improve diving from watching the videos. Sometimes it's hard to understand comments from other divers without witnessing your own performance with you own eyes. Videos help with that.
Signalling equipment that is always accessible- I have an unbreakable mirror and a whistle on a necklace under my wetsuit. In a worst case scenario, I would not want to ditch my BC, CESA to the surface, and find that all my signalling equipment was attached to the BC I left at the bottom of the ocean.
Besides all that was mentioned by the others (o rings, straps etc) , I also bring a few hoses, wrenches for some basic repairs to my regulator system. Extra batteries for dive comps and torches can be very useful too. I also put in some zipties , mouthpiece, and extra fin straps.