I don't really get a lot of joy out of building copies of things other people have done better than me before. Almost by definition, no list of cool toy projects is going to appeal to me for that reason. I might like the techniques or algorithms employed within them, but frankly if I'm after that kind of knowledge I just find Leetcode and traditional studying to be both higher value and more fun.

But I do like writing programs in some strange new, non-CS domain where very little software to address my very particular needs already exists. Most of the software I've written in the last few years has been of this kind, for the purposes of learning Finnish. It is amazing what a few shell scripts, fzf wrappers, and well-timed calls to an LLM can do for accelerating one's own learning process.