My learning / hobbies turned around a bit. I now like to read stuff about kids, their mental development, hoe to stimulate their growth etc. I love it.

Yes, it's very different from the hobbies I used to have, but for my brain, making the kids a hobby/'project' works pretty well.

I do kinda agree with another commenter here that it does somewhat sound like you simply need more challenging / interesting work. My four day workweek is fun / challenging enough that I don't really need to do anything similar (i.e. write software) as a side project.