I haven't worked on them in a while to be honest, but for many years I worked on a small snail simulation in my spare time and blogged about it[0]. The project had gone through JS, PHP, and later Go iterations. I also spent a few weeks playing around with implementing an approach described in this paper on open ended simulations[1]. Go was not the best language to choose for this project[2], but it definitely counted as fun hobby coding for me.
[0] https://liza.io/categories/snails/
[1] https://www-users.cs.york.ac.uk/susan/bib/ss/nonstd/ecal11-1...
[2] https://liza.io/roee-self-modifying-go-simulation-experiment...