Sharpie Paint Marker
This one may not make much sense to loved ones when you send out your wish list, so you may want to pick these up yourself, but they’re great. For one thing, they easily mark tools, much like a golfer marks balls, and a wrench needs to keep track of tools. At $3 each, they’re perfect for marking saddle heights and other fit points as well. I’ve found the medium point marker perfect and it comes in a rainbow of colors.
I also use them to mark my tubes. Before I stuff a tube into my pack or a packet, I wrap it in electrical tape — which can later be used to boot a tire or fix something rattling on your bike — and then I write the tube size on it.