I have very recently discovered the fun of making a pocket letter and have become a tad obsessed with them. Essentially they are 9 ATCs (Artist Trading Cards) on the front, and instead of doing the traditional ATC back, you instead write a letter, lists, quotes or other small thing on the back.
You then pop them all into a trading card pouch, add a few trinkets, fold the entire thing into a third, and post in a standard sized envelope.
Simple and so much fun! This is a Christmas themed one sent off to a penpal.