Not really a gift idea, but...
I bought a special birthday hat for my daughter's 1st birthday and she has worn it four years in a row. I LOVE the pictures of her in the same hat each year and it REALLY shows us how much she has changed.
I also keep a baggie with an invitation, list of everyone who attended, pictures, etc for each birthday.
I write letters to each of my children on his or her birthday describing the year.
We give each child a small gift and put money in a college account each year. They get so many toys/outfits so I try to do special, meaningful things instead.
I am not one to cry often, but I cry EVERY year when people sing happy birthday to my children! It's so emotional to me to see them growing up and things changing....enjoy those moments!