It might be wise to take a copy of your ferry ticket and keep it in the car with copies of your insurance documentation as well.
That way if stopped by Traffico you can prove how long the car has been in Spain and won't have the car impounded.
Ensure you have two emergency triangles plus spare bulbs in car. If drving through France I think you also have to have a fire extinquisher. You should also carry a medical kit and a safety jacket for every seat in the car even if you're the only person in it.
Do not use your mobile phone in the car even if stationery. If you have to use it get out of the car. Not sure if this applies to a passenger but it does to the driver.
There are also people around who target foreign plated cars so be careful and put your money and documents in a safe place.