Equipment to carry in your one bag:

An immersion heater coil (dual voltage is best)
A 1-quart small-mouth thermos with steel cap/cup
A spork, in case your hotel room doesn't have utensils.

If you're not familiar with thermos/vacuum bottle cookery, just Google it and you'll find several articles. Basically, you put your food in the thermos with a little salt, add boiling water, and the food slow-cooks overnight while you sleep or during the day while you're out. When you awake/return you get a hot fully cooked meal. Eat it in the cup with the spork.

I've done this with whole wheat berries, whole oats, and brown rice and beans or lentils. You can make a whole stew this way by cutting the pieces small.

Works great. Tastes great. Very nutritious. Cheap. Fast preparation. Food stays hot a looong time.

Of course the thermos serves double duty as a water bottle. You get the food at the local market. Boil water in the steel cup so you don't crack the hotel room drinking glasses with the boiling water (which I've done). The coil fits in the thermos and takes up very little space in your bag. You can store other loose items in it as well. A small mouth thermos retains heat longer than a wide-mouth.

Ooops! I forgot one important thing:

After you fill the thermos with the food and boiling water, lay it on its side -- the food will cook much more evenly than if the thermos is standing up.

We used to do a similar thing while traveling in a van. We would heat up beans or whatever in the morning and turn them off while traveling. when we stopped for lunch or dinner the beans (or rice would be cooked. One of our friends cooked rice this way in her coffee cup while hiking.

Forest

This intrigues me--but I've had the tops from several good Thermos' (i.e., Nissan, et. al.) go belly-up. They seem to be the weak link. Does your thermos use standard "pop-up" tops that you can pour through or some old-school kind of plug?