Draw a picture, bookmark, refrigerator magnet, photograph, decorate a playing card (add glitter & stickers on the queen of hearts, joker, or ace of hearts), play tic-tac-oe on a separate piece of paper & send back n forth, make your own puzzles (jigsaw puzzle from a photograph or old greeting card, word search, unscramble words, etc), couple sticks of gum, start of a folded gum wrapper chain, pkg of candy such as gum or pop rocks etc., recipe card with your favorite recipe & set a day that you each make it on the same day, penny, nickle, dime & quarter with the year she was born, postage stamps or SASE, pressed flower, handmade business card, return address labels for her sending letters back to you, homemade stationery.
I also found this website that might be an interesting read....
http://www.homemakingcottage.com/lady/handwritten_letters.htm
Have fun & enjoy!!