Try some challenging letters:
A letter from you to your main character.
A letter from your main character to you.
Not so much a letter as an article:
Write an article as your total opposite would write one. For example, since I am an elderly sinner and a progressive thinker, I would write as a young conservative woman who hates to write. Maybe an article about political views or social views
Write a one pager that must contain two words: “I” and “writer” You can have plenty of other words, but you must have those two words on the page.