Easy and adorable cake decorating idea! Fill a cookie cutter with sprinkles, it doesn't have to be a number. Just pick it up! (You can make a shape that's "faded" at the edges by filling the cookie cutter up to the top and lifting it up quick.
Have guests write message to the birthday boy/girl. Could do a new book each year as a keepsake. I have the Places you go for my daughters school years but the birthday idea is super cute too! I didn't know Dr. Seuss had a birthday book!