While you are thinking of spares one of the most common failures in dive equipment is the BC inflator. Either the rebuild kit and the tool or a spare unit makes sense.
And not just spares, equipment reliability is a choice as well, on my trip to the Red Sea, knowing there would be many dives and rugged conditions and sometimes no service or stores to purchase or rent gear, I chose not my prettiest prized regulators or my lovely antiques or even my most expensive fancy brand new stuff. Nope, I chose instead a pair of old G250s and a new Mark 2 Evo. A rebuild kit for both stages and a third spare G250. A Mark 2 Evo is bullet proof and a G250 is robust and reliable and both can be serviced with minimal tools.
In fact, the G250/Mark 2 Evo combo proved to perform so well I took it to Cozumel just a few weeks later again as my main reg set:
That DGX BCI there is not part of my normal kit. I just bought it for a specific purpose and I was trying it out to see how it did with my wing/BP rig.
A funny, my first dive with the BCI, I back rolled in and my mask slipped off. I grabbed my mask before it escaped and with my other hand clipped off my camera tether to the crotch ring and being distracted, I guess I missed. I then let go of the camera to futz with my mask. I then noticed upon clearing my mask that my camera was now 20 feet away and going fast. No problem, dump my wing and I will grab it, nope! Being unfamiliar with the feel of the BCI I pushed the wrong button. Oops! Now the camera was much further away
. Now, I pride myself on never arm swimming. I maneuver 100% with my flippers all the time. But I was arm swimming, fin swimming and any kind of arm flapping swimming I needed to catch that dang camera. And of course it was observed by a diver who I had just suggested that they keep their arms at their sides or crossed, so it goes
. The point being, when far away, maybe bring stuff that is proven reliable and that you are very familiar with.