I love L-Systems! Really simple concept and implementation, but a huge variety of results and extensions.

In parallel of my studies, I created some years ago on the side an interactive app for generating and coloring L-Systems : https://epholys.itch.io/lsys . It's a bit rough, but I created a lot of interesting trees (all here : https://imgur.com/a/0Rx7uln) and included them as a zip file alongside the app.