Thought of this while eating a Pop Tart with my son
Have your students bring in their own Pop Tarts, you provide the plastic knifes.
1) Start With a Pop Tart Quadrilateral
2) Ask your students to make a Pop Tart Pentagon by cutting off a corner (and eating it)
3) Now make a Pop Tart Hexagon
...and so forth till you reach a decagon (send me pics)