He'll write me a letter then fold it up in some creative origami way and leave it for me to find: under my pillow, by my bedside, tucked in a book, all over the house. Sometimes with a piece of precious candy from his treasured Halloween/Easter/Valentine's Day stash but he leaves it somewhere then sits back and waits for me to make the discovery.
I can't tell you everything that's in them, that would be cheating, but the one I found this week was one of my favorites. I'll post a quick excerpt (and remember whose spelling I'm using):
You might think at times that your not a very good mom. But I don't think even a person with an all-seeying eye could find a better mom than you (even if we don't have video-games).
I'm keeping this to pull out on days when he's not as happy with me or when I'm not as happy with him--these things get you through the less pleasant parts of parenting and makes me remember why I wanted to be a mom.