As a Godparent, you would be responsible for the spiritual wellbeing of the person being christened.
Suitable gifts for a child might therefore include a children's Bible, a St Christopher pendant, or a cross on a chain. For very young children, these would have to be put away for the future, but then, being a Godparent is all about the child's future.
These gifts are also suitable for an adult, although a different edition of the Bible might (!) be preferable.
It's generally a good idea to leave the religious theme to the Godparents - you don't want to get the same as them, after all. Another possibility is a moneybox for future savings.
How about a special boardgame - there are some beautiful Peter Rabbit and Pooh Bear games available.