If the kid likes plane geometry and is interested in further math, I’d highly recommend Yaglom’s books Geometric Transformations. They are a series of (hard) pr

If the kid likes plane geometry and is interested in further math, I’d highly recommend Yaglom’s books Geometric Transformations. They are a series of (hard) problem-focused books which teach the ideas of transformation geometry in service of solving various construction problems. In general transformation geometry is drastically underemphasized in American (and possibly other countries’) secondary and early undergraduate math education.