Bridge challenges. Physics challenges- "Build the widest structure with only two planks touching the ground." Simple parameters with unlimited solutions. Students must expirement with the forces on cantilevers, fulcrums, balance, triangulation etc. Explore the golden mean, optimum proportions.
Structures can be incredibly complex. Probem solving, teamwork, creativity happen automatically. Students feel like, engineers, inventors and sculptors. The level of concentration will impress you. Exceptional children build exceptional structures.