I've been working on a HTMX Playground!
https://lassebomh.github.io/htmx-sandbox/
It runs a small mock server inside the browser which is very Django inspired, so it should be very familiar if you have experience with it.
The mock server runs using PollyJS and Nunjucks as the templating engine. There is sadly no mobile support or the ability to save yet, but it's still a great place to start tinkering without needing a full setup.