I use this activity as a mode of writing assessment; however, it functions well as a means of getting a student taking about their summer and their thoughts about starting the school year. In addition, it allows a student to clear the plate of whatever is on their mind before starting a new school year...

Students must follow the guidelines of a friendly letter and write a letter to themselves. This letter should address their summer activities, their feelings about returning to school, their thoughts about this coming school year, their goals as well as anything non-school related that is on their mind.

I give the option of allowing the students to submit the piece and simply giving credit for it. I do not grade these pieces or even read them unless the student asks me too. That tends to make them a bit more comfortable and eager to write.

When all the students are done, I take the letters, seal them in an envelope with some sort of candy and I put them away until the end of the year. On the last day of classes, I hand them back and let them read them.
The students get a real kick out of the letters and invariably share them with me.