What are unconventional writing exercises that can dramatically improve one's skills as an author?

Write about scenes from a TV show or movie.

Pick any scene that resonates with you, not just as audience, but as a potential author. Take notes about the characters in the scene, like their gestures, their outfits, their mood and especially what they're saying.

Then write the scene down, exactly as it has happened. Putting that one constraint on your writing will force you to work with what is already a good enough scene.

Make sure you include only the details that make the scene stand out to yourself, and not the trivial details that contribute nothing to the pace of the scene.

This exercise will show you how good you are as a writer. Because the scene is already good enough to catch your attention. Read your work and compare the effect of reading it on paper with the effect of watching it on a screen.

It will teach you which details to include to make a scene more effective. You will also find how to incorporate gestures and body language with dialogue.