Keep the car stocked with a stroller of course, but as your child gets older, add in a clip on booster/high chair seat and an extra potty or potty ring.  You never know when you'll need these.  A friend's house who doesn't have kids.  A restaurant.  Etc....

Another good idea is to keep a small bag in the trunk packed with wipes, diapers, baby tylenol, an extra bottle or sippy cup, and a change of clothes for both you and your child.