I found this idea recently in an article I had clipped from Parade magazine in 1998, when my oldest daughter was not even a year old: A Message from the Child You Were.
Have your child (the author recommends 9 as a good age) write a letter to him/herself which she will open when she graduates from high school. She could include things like hobbies, favorite foods and TV shows, good friends, hopes for the future, etc.
If your child hand-writes the letter (and perhaps even draws a picture), it will be even more special. Seal the letter in an envelope and store it somewhere where you’ll be able to locate it when your child is 18 (I know – that could be a challenge!).
My third and fifth grade daughters have done this already, and I plan for my second grader to try it in the next year or so.
The author of the article was a teacher who had her students do this each year, and then she would mail the letters to the kids when they reached graduation age. She reported that they just loved reading the letters from “the children they once were”!