Even though this is in the meantime a topic that is 20+ years old: I still find GPU shaders extremely fascinating: A super versatile way of leveraging the enormous power of your GPU for playful recreational coding. Ray Marching is a technique that is also around since ages and that lends itself very well for being implemented on a GPU. Using Ray Marching, you can achieve stunning results with just a few lines of code. The Art of Code is an inspiring YouTube channel, which I used back in 2019 to learn how to do Ray Marching. One of the godfathers of the topic though is Inigo Quilez, whose website offers a wealth of material on the topic.
Here are two experiments I made in 2019 using Shadertoy. Click the images to see them running and to have a look at the source code. Currently, the music that is affecting the Audio Tunnel in realtime only works on the Firefox browser.
done by sy2002 in March 2019