Bring a leash. It may be required at rest stops, and you never know what even your well-trained dog will do in a strange place, amidst other pets and people, next to a busy highway. Your cat is very likely to think heading back home on foot is a good idea. It can be very difficult to hold a frightened cat, and nearly impossible to catch one that has escaped. Consider bringing a pet carrier.
Motels are more likely to allow a pet in a carrier.
And if you put your cat into his carrier before you open the car doors you may prevent an unauthorized departure. Bring water, especially for old dogs or cats. A nervous panting pet can lose water rapidly, and dehydration can be a serious, even life-threatening problem. If you are planning to board your pet at a kennel, make reservations early. Ask what vaccinations and other health care the kennel require so you can have your regular veterinarian take care of these things and make sure you have proof of things that are required.