If you can have a headlamp addiction I do. Every bag has one and most have a spare.

That said I keep 2 mini cyalumes in my edc first aid kit. They each are the about the size of a AA battery, weigh only grams, require no batteries, if one should somehow snap I have a spare and most importantly, I have no problem giving them out and never seeing them again.

I've also used they for marker lighting.

You can heat them in hot water and get a much brighter light, (with a reduced total use time).