I actually heated up a rice ball and a canned coffee with the "Morian Heat Pack Heating Set!" Hot food and drinks, even in an emergency!
The "Maurian Heat Pack Heating Set" is sold by Kyodo. This set includes a heating agent and a heating bag, and is a convenient item that can be used to heat cold retort foods and beverages anytime and anywhere by adding a small amount of water. We actually tried heating it up.