We have done this project with the younger classes as well. My spin on this is that the students had to add something unexpected to the sandwiches. Can you find the unexpected... toothbrush, fruit, pencil...
Another student explored Yupo paper for watercolor and did this modern Mona Lisa. Yupo paper is a plastic type surface that has quite a learning curve but fun once you know what the paper will do and what it can't do.