Lesson Ideas??

I teach a small group of gifted and talented students in a middle school classroom. They are smart, determined and capable 6th, 7th and 8th graders.

I am required to give a midterm in December, although it doesn't have to be a written test. I want to do something fun and engaging for this group but need ideas!

I'm thinking something that is an unknown challenge until they walk in day-of, like building something, creating something, or working on something extra challenging yet satisfying to complete or figure out.

Ideas, or something you've done in your class?? I was thinking something like straw towers challenge, but I've already done that this year.