One of the teachers I work with, Bec, put together a great unit of work around the book "The Jolly Postman" by Alan and Janet Ahlberg. Students made story maps of the journey that the Jolly Postman went on as he delivered his mail, and learnt how to write a letter and address an envelope.
Bec also made up a simple A5 letter template for students to use to write letters to one another. We made up red post boxes (as our postboxes are red in Australia) and encouraged students to start writing. I gave my students a folder to store their letters in so that they remember to reply to the people who write to them.
It has been amazing to see how involved students have become with this. One student even went home on the weekend and wrote letters to just about everyone in the class! I was surprised by this as it used to be hard to get him to sit still and write at all. As you can see, this project really has the kids motivated.
I have also been impressed by the quality of the letters. Students are trying to write small and neatly. They are also checking punctuation (with some reminders). Most of all, I love the content of these letters - they are using the social conventions for letters, and as time goes on, are making the letters unique to the person to whom they are writing.