That reminds me of something I've been thinking about again and again over the past years. Is there any reliable way to send myself a message that I would receive, say, 40 years from today. So far I haven't been able to come up with a good solution.
Get a locket engraved with the date. Write the message on thin paper, roll it up, and store it in the locket. Wear it everywhere until it's time to open it. Keep some spare copies in a safety deposit box. If you lose the locket, you'll notice, so you can go and get another one from the box.
// Prepay for 40 years, leave the message in the box, forget the lockets. The bank will chase your for an extension for the rental of the box.
Far from foolproof but if you're already relying on a deposit box ...