Vintage Record Sleeve--you can find orphan record sleeves in all kinds of places, maybe even in your own collection. I've seen them in book resale shops, thrift shops, and estate sales. Find a cute one, add your letter or paper bits, seal with tape, and send it! I used vintage Dennison labels to address mine.
Or try this--a vintage children's record/book. Write the letter on the pages of the book, and fill the sleeve for the record with little treasures, tape closed, and mail. Who doesn't love a memory of a good Disney story?
Adding vintage ledger paper to the inside and securing with washi tape makes a great place for your letter. And it's kind of like a treasure hunt through the book to find the letter! This idea was from Xenia of Saturday Morning Vintage.
Vintage Bound Photo Album--There are almost always old pictures at the estate sales we shop, and I've picked up a few old photo albums from family trips. Write your letter on the back of the pictures, and let the photos be a trip down memory lane, or if they are not your own, they are cheap entertainment and the subjects remain anonymous!
An old library book makes a great presentation. Xenia, of Saturday Morning Vintage, sent her last letter to me using one. She wrote the letter on ledger paper that was taped to the pages using washi, but the pictures of the birds in the pages were left untouched for me to enjoy. This was one of the cutest things I have ever received, and I love her letters. She taped it shut and put the postage right on the cover of the book. It is beautiful!