Open the leftmost glass cabinet door. It’ll give you an empty inkpot, and chocolate!
Click Lupin for more info about him! Also check behind the door for a roller… most likely left by the previous inhabitant of this office. Let’s at least assume so! Unroll one of the scrolls, and pick up the old quill. Then play with the door to finish the moment.
New from J. K. Rowling:
pick up the exploding bonbons, pumpkin pasty and free galleon
Last but not least, you can also pick up the Gobstone, which is rolling around.
And that, my friends, is the end of Prisoner of Azkaban.