So one of my essentials for travel is an Immersion Heater, I stick it in a glass with some water and in a few minutes I’ve got hot water. It’ll boil water if you want, there’s no moving parts and it’s really easy to use. I think it’s just for water, some bad reviews talk about other liquids, but I’ve never tried it with anything but water; I’ve never considered anything but water.
You can use it for heating up water for tea or coffee or whatever. 
I initially got it for heating up water for my Netty Pot, sometimes the hot water when traveling isn’t warm enough (or I don’t want to use tap water if it’s somewhere I wouldn’t drink the water).
But since I’ve got it with me, sometimes I’ll use it for heating up some instant soup or Ramen-in-a-cup for a snack when traveling. I’m trying to think of what else you can just add boiled water to: instant oatmeal(?), instant soups + noodles, instant potatoes (but that sounds boring), Mac and Cheese (I think Kraft makes a just add hot water?) and if you had a fridge to store them, you could probably boil an egg.
You’ve got to remember to unplug it before you remove it from the cup of water or it’ll burn out (or something) and watch it when you’re using it because if it boils all the water away it’ll stop working permanently. The one I’ve got has lasted a long time but I’ve let it boil down pretty far before when I’ve forgotten about it; I actually travel with a spare, just in case.
