Whenever I get bored (rarely these days) I like to write path-finding & annealing algorithms. I start with a super simple ray-tracing framework I first wrote in the 1990's for visualization, and then I play around with things like gravity and electromagnetic repulsion / attraction. My favorite thing to do is watch a million charged spheres distribute themselves on a weird surface obeying gravity and electromagnetics, and then have another sphere try to navigate through them. It's pointless but satisfying. I'm sure Blender and Processing and all the new fancy tools can do it better, but I understand every single line of code, so I feel like it is something I made for me.