I am a godparent myself. I've talked to my priest about this. The role of the Godparent really, is to make sure the child grows in his faith. So that means prayers, and when you can, teach him about the faith when he's older. So basically, a decade of the rosary devoted every day to your godchild is the greatest gift you can give. I suppose you can think of something to give him materially, I guess, on his birthday or Christmas.