A Time Capsule Santa Letter is one of the neatest heirlooms that we have implemented in our household. Every Christmas, the kids write or attempt to write their letter to Santa after meandering through the current Fleet Farm catalog for endless hours. We try not to focus too much on the material part of Christmas, but we still do the whole Santa/Saint Nicholas thing secondary to the "true meaning" of Christmas.
After we have written our letters, we sign and seal them, and the kids race to the mailbox and stuff their letters in. Then, when they are sleeping that night, I sneak outside in the chilly air, open the mailbox, and snag their letters. The kids think the letters are on their long journey to the North Pole, but really, I have taken them and stuck them inside a special envelope locked up and sealed. Not because I'm the Grinch. but because when they are older, I plan to hand them their letters in a binder (and hopefully they can treasure them forever....without their own kids asking why Santa doesn't have the letters :)