Here are a few:
- Color the INSIDE of the envelope
- Make the paper a cool shape (see here for examples)
- Send a puzzle (see here for example)
- Send a week long package (see here)
- Send a hug
- Send Polaroid notes (the example I saw isn't exactly what I was thinking of doing, plus they're out, but to see it go here)
- Ransom Note Style (apparently there's a kit for this, because cutting your own letters out isn't cool anymore, you can buy that here... I will be cutting my letters out)
- A postcard with an envelope glued on back (so cute, example here)
- Word picture letter (kind of like this)