If you plan to park your car at an airport for more than a day or two, fees can add up quickly—even at the discounted off-site lots. And even if you don’t leave your car at the airport while you’re gone, your car just sits somewhere being of no use to anybody while someone you don’t know arrives at your hometown airport and rents a car, again for a stiff fee. And maybe you’ll need a car rental at your destination, too.