Guest Books on your 80th birthday aren't just for signatures. You really want people to write something thoughtful about the person celebrating their 80th birthday. That way you can always look back at the celebration with warmth and happiness, knowing that you are much loved by everyone who came to celebrate with you.
People can surprise you when you ask them to write something for an occasion like this and they can dig out memories of earlier times that you had either forgotten or put to the back of your mind. They can make you laugh, or cry, or smile, or sigh, but this is a gift that only your friends and family can give you collectively, and this can only happen if you have your guest book ready for them on the day you celebrate your 80th birthday