One thing that's not directly related to ray-tracing proper, so makes no sense in this book series, but is still a pretty natural (and fun!) next step (or detou

One thing that's not directly related to ray-tracing proper, so makes no sense in this book series, but is still a pretty natural (and fun!) next step (or detour) is embedding e.g. a Lua interpreter so you can use that as a scene description language separate from the rendering engine proper.