I have received a “Thank you” note with 20 USD tip once.
My wife (she’s also renting out an apartment on her Airbnb account) once received a long letter from a guest saying “you are so beautiful, I adore you, I want to book again just to meet you once”. The cleaning service found it and was very amused. (By the way, she has never met the guest, it’s self check-in and self check-out.)