No, you should not leave dry ice outside to melt alone. Dry ice is extremely cold and can cause severe frostbite if touched without protective gloves or clothing. You do want to have it outside or in a properly ventilated area because of the toxic fumes that are released, but make sure no one will come in contact with it. Additionally, it will evaporate quickly in the open air, so leaving it outside would be a waste of money. If you need to dispose of dry ice, do so safely by placing it in an insulated container and allowing it to sublimate away from people and animals.