Lots of good answers here.
Another option would be taking MRE-s (meal ready to eat) it's pre-packaged food designed for soldiers to eat while not having access to a kitchen. It usually has a solution to heat the food and plenty of calories for a full day.
You can order them on the Internet from military surplus or other places and there is a bunch of flavours to choose from. They also have a long shelf life, don't need refrigeration, and fit in a small space.