If I was receiving a letter I would want to hear . That you love me and that I make you smile and how happy you are to be with me and when your not I'm on your mind and in your heart . Idk just tell "him" how you truly feel speak from the heart.
I'd want one that was truthful, not overdone. Otherwise it looks stalker-ish. Like "I want to be with you every minute of the day" is just... no. Some guys like that, though I don't see why. Keep it short, nice, and truthful. A simple "I love you this much" card with a kitten with paws stretched out is nice. 'Course, again, not all guys like or dislike what I do.
I would like to hear about passion, love, sincerity, intimacy, what she wanted to do with my he he and what I could do with her who who, and talk about her what nots and her naughty bits, and what kind of delicates she was going to wear when she saw me, you know the usual stuff...)