Pick up a 100th birthday guest book and provide your new centenarian with a gift they will cherish, and will move them to tears. It isn't about collecting signatures, it's about collecting lovely messages from those people who think the world of the birthday boy or girl, and each finding a slightly different way of committing those thoughts and feelings to paper. It's best to get the guest book in advance and send it round to everyone so that they can add their contribution, in whatever form it takes, so that you can present the gift on the big day. Then just wait to see the reaction you get! You'll be delighted that you took the trouble, and so will your guest of honour.