Guidelines posted on Kristina's website make some suggestions:
Some ideas include...an actual WWII letter; an imaginary note from Juliet to Romeo, from Donald Duck to Daisy, or from Amelia Earhart to a secret love with whom she might have run away. It can be a letter you once received, a copy of one you've sent, or one you should have written long ago. It can be a message to God, an estranged relative, a deployed serviceman, your favorite celebrity, and the list goes on.Go ahead. Write your heart out!