Have you seen the OPC series of CPUs? (One Page Computing - the challenge being to keep the code small enough to be printed onto a single sheet of line printer paper!)
Building a custom CPU commits you to writing an assembler and listing generator - which is a good hobby-project job for one person who's handy with Python. After stumbling through those foothills, though, I found myself at the base of some very steep, scary GCC/binutils cliffs wondering how I could have gotten so lost, so far from home.
Even if all RISC-V does is offer a bunch of arbitrary answers to arbitrary design questions, I consider it a massive win.