I'll add Flexeez (US name) or Wammy (European name), a geometric construction toy originally from Japan, I think. My 10yo figured out all of the Platonic solid

I'll add Flexeez (US name) or Wammy (European name), a geometric construction toy originally from Japan, I think. My 10yo figured out all of the Platonic solids using it, and then moved on to hyperbolic shapes. Also made lots of hats and belts and bandoliers, etc.