I have seven God children, I have opened an individual bank account for each of them. I deposit a bond for their sacraments like christening, communion etc and for each birthday and Christmas, I deposit an amount.
Everyone godchild will receive it at their college graduation one day.
In my eastern eauropean roman catholic tradition is for the godmother to buy the christening outfit. We don't do crosses on necklasess as gifts, angels or other saint figures are more popular.
A nice heirloom keepsake is a personalized small pillow, picture frame with a picture of three of you. Since she is older, maybe a jewelry set or
personalized jewelry box ( pottery barn kids has a beautiful collection)
Congrats, being a god parent is honoring and a flattering role. The greatest gift to your god daughter is to be there for her and to be a positive example.