If you mean for injuries etc. then some sort of absorbent lint or padding, a bandage, some vetwrap, antiseptic wipes, water, a bit of honey and be prepared to carry your dog back to the vehicle if he goes badly lame.
In the car if you're a way from home or veterinary help then also carry icewraps (migraine patches are handy as they can be applied and bandaged into place), a bottle of disinfectant for wound washing. More bandages, cotton wool, absorbent pads etc. I always carry old razor blades just incase you need to shave a bit of the dogs fur off to get better access to the injury site.