I've been developing Rails apps since 2006, and I even built a simple Rails clone called Dubious in Mirah. This was useful for scaling Rails apps on Google App Engine for Java. Sometimes, I just like to carry a smart feature phone. I created Caltrain apps for KaiOS and J2ME. The KaiOS version is a PWA written with VanillaJS.
Additionally, I like to implement algorithms in several languages to refresh my memory on syntax and features. I've also created some obscure Docker images like nanoserver-ruby and geminabox.