Wada Basins are fractal patterns that emerge from repeated reflections between closely nested spheres. The simplest and most well known version is to use 4 spheres stacked into a pyramid shape (3 forming the base triangle supporting the 4th sitting on top of them). When you look into one of the gaps you get Wada Basin patterns.
This sort of setup is ideal for ray tracing. I recently started to code up a simple ray tracer and this is the initial result of simulating a Wada Basin setup. 4 reflective spheres colored red, green, blue and white with a single point light source.
Next Christmas, grab 4 sphere shaped ornaments off the tree and make your own fractal patterns.