It might be worthwhile for grandparents to keep a notebook recording their experiences when grandchildren came to visit/stay, particularly when too young to record anything themselves. It could be kept for when they were older, and make interesting reading, particularly if honest, and not just a rosy picture - temper tantrums, refusing to eat grandma's food, not going to sleep! (not that my perfect grandchildren would be like this
).
Photos could be included, and examples of crafts/pictures child did while visiting and also what their favourite stories and songs were at the time.