I think it depends a lot on where you are diving and how long. I generally like to bring an entire spare regulator with computer and all hoses etc. Then any failure on my regulator can be handled by some channel locks.. more or less.
Maybe a mouth piece thrown in a box, maybe a spare light. zip ties are always nice, a spare bolt snap, some string, fishing line or cave line for misc issues. A couple spare tank o-rings, often a spare mask, sometimes a spare knife and I often throw one extra glove in the gear bag - might have to wear it backwards.
An allen wrench for port plugs and some pliers and usually an extra speargun.
If it is a multiday trip, I often bring an extra freedive fin, in case one blows out of the boat or something.