Once you finish the main book, there's a few free bonus chapters provided [1] that help you implement soft area lights, bounding boxes (AABB in this case), and texture mapping. Beyond that, you might be interested in the "One Weekend" series [2], which is all C++ but is mostly general enough for other languages. There's also PBRT [3] which is an open source textbook/documentation for the pbrt-v3 renderer [4], but this is beyond the scope of "small exercise" territory. Good luck!