I think, since it doesn’t say that letters can be sent back, I would write a very detailed account of a day and send it to myself. Sure journaling would produce the same effect, but I’m better at writing letters than consistently journaling. However my other option would be to send silly letters to my friends (and family). A letter that represents who I am right now and what our relationship is at this point in time.
If I got a letter when I’m 40 or 50 (or whatever age really) from one of my friends as they are now I like to think I’d cherish it. Right now we all in such a period of transition (bridging that gap from college graduates to adults with full time jobs) that is changing who we are. Not so much changing I suppose, just forcing us to evolve into the people we will become. Who wouldn’t want a letter, once they have become that person, from another person who is living through the transition? I think it would be quite interesting. (And if you were able to follow that muddied sentence give yourself ten points for the day!)