One quick comment: is there an open-ended sandbox where I can just write whatever code where I want? Even after creating an account I couldn't find such a button, or even the "pay $5/mo upsell" button.

I feel like such a mode would be useful for instructors that are working through a problem with their students, ideally without having to go through all the lessons first.

As for students, once they get proficient in a language they'll want to use it to make things -- from what I've seen in other learn-to-code environments kids end up co-opting the later lessons to do so if you don't give a way for them to create new projects.