I put my students in groups of three. Each group received 50 plastic straws and one roll of transparent tape. The instructions were simple: build the tallest freestanding tower.
Students tried a variety of structures. Some were quite tall, but they could not stand freely.
Finally, students in one group stood back and looked at the progress of the other groups. "Everyone is trying to build the tallest structure, and none of them will stand," they commented. "Let's build a shorter tower that can stand on its own." They did it, and guess who won?
I recommend this activity for a fun end-of-year activity - - - or anytime! It really gets kids thinking.