A cheap Jeweler's loupe from Amazon. It's like a high magnification magnifying glass with a LED light built into it. I initially bought it to inspect my kitchen knives when I sharpen them, but it's been useful for inspecting soldering work and other random little things.

Others have mentioned a powerbank and one of those multiport USB chargers. I'd second those as well, tho I guess they both can climb in price depending on if you need a lot of power capacity.