I think Implementing a 6502 is a good example for an architecture implementation. Since it has already been done in numerous NES emulation projects, A lot of us have a good understanding of the CPU. Plus it is the part of a history which many kids could relate to. The video game era and genesis of personal computing era was almost entirely based off of this single CPU. It isnt small but definitely a stepping stone towards understanding a modern CPU. anything lesser makes it just a toy. For a toy, I like the one described in "Elements of Computign systems" here https://www.amazon.com/Elements-Computing-Systems-Building-P...