When learning about nutrition, students often want to know how "they" figure out how many calories are in a food. Rather than explaining it to them - let them do it themselves! Students especially love getting to burn things in Science class.
This version of a Chemistry classic has students build their own calorimeter using mostly materials that are easily available at home rather than a "real" calorimeter. They burn a marshmallow and a walnut (careful for nut allergies) and determine the caloric value of each item.
You can also explore measurement error with the students, since their calculated values will be quite different from those on nutrition labels.