One of the things I like the most, probably at the same level as real-time audio and graphics, is developing creative tools to help designers create real-time content. While I was more involved in the Demoscene, I created plenty of tools to generate [64kb intros], and they made me improve my coding and UI/UX designer skills. I've written a little post for each of the tools, and I hope you will enjoy them:

These tools have been used for some of these Demoscene productions.