Get something thoughtful. Last year I went to Michaels and bought these concrete stepping stone kits that you put the kids hand prints in. You put the date on them and can decorate them with cool little tiles. They last pretty much for ever and you can do a new one each year to see how they have grown.
For more information and a store near you: http://www.michaels.com/art/online/displayProductPage?productNum=gc1317