I'm envisioning a double lesson here. Proportions & Volume/Area of Irregular Shapes (with a built in lesson, if you would like, on choosing the right quantity of food per serving).
Foremost, have your students bring in a happy meal box sometime within two weeks prior to the class, labeling everything that came in the box on the inside of the box with their name. You may want to swing by your local McDonald's and ask for extras for those who forget (don't forget to bring your teacher badge).
Next, you can go here and figure out the calories of each item. Or download the pdf