If you have to carry luggage on the roof, use a roof box to reduce drag. Alternatively load luggage on a roof rack as low as possible and wrap tightly in plastic sheeting.
Opening windows will cause extra drag that will cause the vehicle to drink more fuel use air vents first particularly on a motorway. Once air conditioning has cooled the inside of the car, you may be able to turn it down or off. Don't use the air conditioning if doors or windows are open. Increase tyre pressures if carrying extra passengers or heavy luggage. Using a windscreen shade and opening up the car as soon as you get back to it will help to cool the inside. Opening windows while you drive out of a car park will lower the inside temperature before you start the air conditioning.