"So I need to write something. First-person POV, short story, something I don't usually write... and interesting."
In the early 20th century a lot of mystery stories were written in first person POV like that. How about a short mystery? Maybe you could describe one of the magic-related cases Cas has to handle--like he's sitting in his office and someone walks in and puts him on a case, so you're writing from his viewpoint as he's trying to solve the case.
"But before I do that, let's procrastinate and spend money buying those Card books..."
That's one of my favorite ways to procrastinate :)
Oooh ooh- a magic gumshoe film noir!