First of all, one can never run out of math activities. There are so many that are never experienced in school books. School still works on the reader's digest version of math.
I also believe that it is up to parents of gifted children to introduce their children to other interests. I remember going to university and looking at the various courses and being upset that I had never experienced them before.
Many other subjects embrace math such as probility and many sciences. My daughter is also learning Latin and Anthropology. We have been with Girl Scouts for six years and she works on badges and other activities that introduce her to fun interests. Some parents just buy the badge books for reference and fun.
