Then give the children a chance to develop themselves further; more advanced tasks, other variations of the same task, ask them to explain to you how they think when they solve the task, give them something completely different to spend 20 minutes learning/doing, etc.
I highly recommend juggling and balancing (a broom stick on the chin is a personal favourite) - it can be taught easily, requires quite different mental gymnastics and helps generics such as focus and determination. Plus circus is all about collaboration and entertainment, things that are rarely taught outside of a competitive framework in school. Good preparation for the development of an "interesting" personality, but I would say that!
