I guess what sort of domains do people like to play around with in their own time? For games there's 3d rendering, physics and AI which seem fun (at least to m

I guess what sort of domains do people like to play around with in their own time? For games there's 3d rendering, physics and AI which seem fun (at least to me) but I'm not sure how to necessarily make fun challenges out of them. For non-direct-games things fancier AI like deep learning, audio synthesis, making compilers/parsers/transpilers, math stuff like project euler, puzzle/board game solvers/bots (https://gist.github.com/christopherhesse/51e9baf0e3d440d8afff https://gist.github.com/christopherhesse/51e9baf0e3d440d8aff...) come to mind. Some of the stuff in the 3rd stripe CTF was pretty interesting like making a search engine or dealing with distributed system consensus problems (raft). Maybe using the coolest new framework is fun for some people, like a challenge could involve angular.js so you could get some experience using it without having to go full-on side project or put it in at work without having any real idea if you should. Implementing things that you are familiar with but that you don't understand the internals of has been fun for me in the past (for instance like https://www.mikeash.com/pyblog/friday-qa-2015-02-20-lets-build-synchronized.html https://www.mikeash.com/pyblog/friday-qa-2015-02-20-lets-bui...). Maybe you always wondered how this thing works, and now you can figure it out and solve this challenge at the same time.