We started a project a few years back: a book, filled with art and letters, that the kids mail back and forth to their grandparents. We call it the Flying Book. We've been through a few of these books now; the photos show our current editions, one for each child. I sewed a piece of their artwork to the paper cover.
Each child draws pictures and writes a few thoughts for the grandparents. Sometimes they ask questions, like, "What do you like best about autumn?" Sometimes they tell them about the art they've made.
The kids often have seasonal themes.