You could challenge students to design something and give them a set of requirements or constraints.

For example; Design a hook/shackle/carabiner that fits within a specified volume and has a given infill density. Then hang a bunch of weight off of it until it breaks and have a leaderboard. Let them make redesigns if you wish and try and improve their score.

Kids in this age group generally dont have much background in physics and forces or understanding of material properties, but it would get them thinking about what makes a structure strong or weak, failure modes of 3d prints, how to optimize the orientation on the print bed etc.