For this lab exercise I originally intended to use larger items like boots and or sneakers to show off depth of field, but found that the concept was much more visible when applied to smaller concepts like these mini cereal boxes. Using the touch focus on each cereal box and watching it gradually push the other out of focus showed the concept being demonstrated; and it proved that the distance between the objects being shot is what really makes or beaks what the main focus of the picture is.
The effect of shallow depth of field will be even more pronounced if you get closer to the subject. Fill half the frame with one of the cereal boxes.