This is a bit trivial compared to others, but it was building a simple compiler for my friends who were working on a video game. They'd written up the scenes in

This is a bit trivial compared to others, but it was building a simple compiler for my friends who were working on a video game. They'd written up the scenes in great detail in a structured format, and I built the compiler to turn those scenes into code. It was super fun because I was working with people I knew, on a project with very little pressure, learning something I'd never worked with before (my background is in Data Science.)