On a boat I take a small bag with some hand tools and a set of o-rings that I might need, a spare reg, some duct tape, a spare timer and acme-instant-snoopy-loop-on-a-roll (gas tight self-amalgamating tape).
Diving out of a car then I take my full spares case with just about everything I'll ever need in it (from years of scrounging and stealing).
Quote: (Originally Posted by
Clare Gledhill)

Small spray bottle of baby bath for bubble checks and seal donning
Absolutely essential. I am very sad to say that top tip won me an AP mug and a d-ring (they pushed the boat out there) ten years ago when ScubaWorld magazine was still running.