It's the Sorcerer's Stone!
It's not entirely chronological, but we're revisiting the story of the Sorcerer's Stone!
Think about it. A dash out to Hagrid's Hut ("I shouldn't have mentioned that"), sneaking past Fluffy, a tall, enclosed vertical chamber (the key room), Devil's Snare, and an escape out.
It'll all be after the events of the book, but the most logical collection of narrative locations.