Here are some other travel supplies that might be nice to have. Others are ones you definitely don't want to be without.
For instance, you should always have a harness and leash in your car, even if you don't plan on taking your dog out of the car.
If you are traveling any distance, you should also make sure you have water and a water bowl. Extra dog food is also a nice thing to keep on hand if you travel with your dog a lot.