Not a car, but a plane... I was probably 8 or 9, acting as co-pilot with my grandfather. We were flying along when he turned to me and said “I’m tired, I think I’m going to take a nap. You take the controls for a while.”

He then proceeded to pretend to sleep while I held us steady on course for ~15-20 minutes. I found out later that he only closed 1 eye, but at the time it was by far the most white-knuckled experience of my life until some puckering moments once I had my driver’s license. Looking back at it now, totally something I would have done.