You can't give gifts like airplanes and cars to a newborn baby. It will take more than half a year for him to play with these. If you want a smart baby, you have to exercise his brain power from an early age. You can enroll your godson in a tutoring class, such as infant courses, how to make babies smart, etc. When the child is a little older, he will see the outside world. You can buy some children's books for the baby to stimulate his visual nerves, or tell her stories. The baby will perceive it.
You can also give a baby bed. Your girlfriend definitely doesn't have time to buy one now. It's better to give the baby a baby bed, which will save your girlfriend a worry, and this is also a gift that the baby can use. You have such a good relationship, you can give some commonly used things like milk powder, diapers, etc.
Whether you give a baby course or any other gift, it is a thought, and she will definitely understand. Don't forget to smile at her when you visit her.