For extracurriculars so far we've done sports (right now it is hockey) and Beavers. He is very average at most sports so it has been a huge learning experience for our DS to actually have to learn and practice something and we often use it to help him empathize with his classmates that struggle with things that he finds very easy. Where we live 95% of the boys (and a few of the girls) at that age play hockey so it gives them something in common especially since none of the other kids we know are going to discuss astrophysics or Fibonacci numbers with him. DH and I are both engineers so building, playing and reading about science/math are as fun for us as they are for the kids and we do a lot of it at home. Most of the cool camps/clubs tend to have much higher ages so we haven't had much choice anyway.
Lego Mindstorms has been a huge hit with both DS7 and DD5. We've spent many hours building and programming various things.
