Scissors, a book, pen and stamps
When the mail arrives, have your child cut the stamps off the letters. Look at the stamps carefully and discuss the pictures on the stamps. Working together, glue them in a scrapbook. Let your child decide how to organize the book. Talk about how much each stamp cost, where it is from, if she already has one that is the same, etc. Your child might want to look up the places the stamps came from on internet.
An exciting family hobby may emerge from this activity.