If you have the cash, something really nice would be a silver charm bracelet that you could add a new charm to each year (until she tells you to stop)
Alternatively, start a collection of something else that becomes "your thing" to give her each year - a friend of mine always gave her godson Brio, for e.g., until the parents begged her to stop cos it was coming out of their ears!
I agree with BoffinMum - time is the best thing that you can give her.
That said my wife is - she's not Godmother, as such as it was a secular "naming ceremony" - I can't remember the term that was used - anyway...
He's our best friends little boy and we buy him books every birthday/christmas, on top of his ususal present. We started out with a little "classics" library - that as well as a few obvious novels/plays, had a few children's classics, and some age appropriate stuff - so a mix of Dickens, Dahl and Donaldson. The idea being that we add things that are age appropriate every year.