I just did letter writing with my students. I have trouble making this fun because it's requires a lot of instruction from me; they get bored listening. I let them choose a person to write their letter to. I had also thought about letting them choose a character from the book we're reading and have them write a letter to the character.
This summer at a conference, the presenter told us about a book that has addresses for celebrities. They had their students choose a celebrity to write to, and many of them responded with autographs and things like that. I just didn't have access to the book becaus I can't remember what it's called