It doesn't have to be fancy. Just say:
"dear person reading this letter,
First, I want to say sorry your job is to read letters. I am sure the risk of paper cuts is high, but I thank you for your diligence in taking that risk and still showing up every day. You are truly special.
Secondly, and the main point of this letter, my last day is on x. In case I am not being clear, this is my resignation letter. I have moved on to better things, but I will always remember my time with your company/school/strip club. Attached you will find a picture of me wearing a sombrero and sipping a Long Island iced tea.
Thank you for this experience that will mean nothing on my résumé, LOR and money (mostly the money).
Love,
anguisette"