Rental cars and gas.

This is more of a US thought as I don't travel internationally.

You're average car will stay on full for a minimum of 15+ miles, some significantly more.

Instead of filling up at the overpriced gas station nearest the airport, stop further out and get it filled up. Save some money and make it much easier. Or, if you're flying out in the morning and you're staying the 15 ish miles from the airport, fill up the night before.