Some fun ideas for writing letters to a granddaughter are to begin writing letters to her prior to her birth or to tell a story about being the age the child is currently. Using postcards or decorative paper can also make giving and receiving letters fun.
Letters to granddaughters can be about casual, everyday matters or they can be more deeply insightful or emotional. The books "Finding The Right Words" by J. Beverly Daniel and "The Gift Of A Letter" by Alexandra Stoddard offer insights into using letter writing as a communication tool with grandchildren. In addition, "How To Write Love Letters" by Michelle Lovric and "The Pleasures Of Staying In Touch -- Writing Memorable Letters" by Jennifer Williams can provide ideas and inspiration for writing letters that are meaningful.
It may be helpful to follow the child's lead, keeping in mind age-appropriate subject matter. For example, if a child asks the grandparent to share wisdom or life stories, then sharing insights that are more profound would be appropriate. Another idea is to share about one's favorite books, either telling about current reading or suggesting well-loved children's books that may be of interest to the child. Keeping in mind the granddaughter's personality is also important. A letter exchange is more likely if it connects with her style of interaction rather than the grandparent's.