I send myself a Birthday Card every Year. Idk why i do, but by the time another year rolls by, i get surprised by the letter with usually 100,000 bells or something like that. I just use it for like Birthday, Christmas, and other random stuff.
I use letters to future me as time capsules. Usually set out as "To me, hope xxx is doing fine, have you done that thing yet?" etc, signed with the date at that moment.