Tape or tie a few ribbons in the car that will flutter within your baby's view but away from his grasp. This will be a whole new type of stimulation for you baby while he is in the car. If he is more than a month old he may be accustomed to the toys on his carrier or car seat and therefore the stimulation factor is diminished. But colorful ribbons flapping in a breeze, well there is something new and exciting!
However, keep in mind that stimulation activities for the car are recommended for quick trips when your baby will be awake. Or you can choose a short period of time during a longer trip. An infant attention span is short and infant stimulation activities should be about 15 minutes and the time for an activity is dependent on your baby's mood and his reaction to an activity.
One important note of caution: if no one will be in the back seat with the baby be very certain that these stimulators are not within your baby's reach as anything you choose to do must be safe for unsupervised stimulation