I started them when Sassy turned 5. I'm not sure if I saw them somewhere or if I cleverly thought of it myself, but it is basically a book that Sassy or Action signs (they each have their own) and hand prints in each year. It also contains that year's b-day invite and signatures, drawings, or notes from their friends and family that come to their parties.
They love decorating that year's page with their hand print, comparing hand sizes and reading back to what their friends wrote or drew in the past years.
See for yourself:
Basic spiral scrapbook (Sassy's was from a local scrapbook store, but they stopped carrying them so Actions is just a sketch notebook)
a title page:
Stamp their hands, have them sign (I photoshopped their signitures out) and let them decorate:
Sassy's 5th b-day
And a few pages from her 6th and 8th birthdays
and a few signatures from Action's:
Great memories of fun times with family and friends.