D.A.
Klutz Chicken Socks has a little letter writing kit called "Little Letters." My dd loves it. It is great for learning how to write letters and has stationary, note cards, envelopes, little notes to fold and seal, stencils to make fold and seal cards, stickers and more.
They also sell a refill pack for it with cards, envelopes and small fold and seal notes.
A diary could be a good choice for a stocking stuffer, also.