For me and my wife, noting that we both dive the same 1st and second stages and the same dive computer:
Dive tool, small box assorted o-rings, fast setting epoxy, duct tape, spare mouthpiece (fits all our second stages and our snorkels too), spare slate pencil, computer battery replacement kit, one spare set of 4 x AAA batteries, one spare set of 4 x AA batteries, spare second stage, spare SPG with HP hose, small folding knife that fits in BC pocket (for use when local regulations prohibit diving with a visible knife), 1/4 sheet of wet/dry sanding sheet grade 500 (for cleaning contacts etc), spare finger spool, spare mask each.
I used to carry spare fin strap & clips until we put springs on all our fins.
Spare fingerless gloves - some locations don't let you wear gloves but will accept fingerless ones. Just covering palms & backs of hands keeps down heat loss.
The fast setting epoxy saved a whole trip when an over-enthusiastic fellow diver pulled my semi-dry suit sealed zipper beyond its end-stop. In a few minutes I was able to get the slider back on and mold a new end stop.