I will never forget the first time I started IEx from within a Phoenix project, and found that I had a live shell into the running application, while the Phoenix development server was running and reachable from the browser, all with hot code reloading... No need to create a dummy REST endpoint to try running a function.
Another thing worth trying, is connecting a Livebook instance (see: https://livebook.dev/) to a running application.
Keep in mind Livebook allows for multiple users to work on the same notebook at once so this can come handy in pair-programming / debugging situations.
My mind was blown.