I remembered a general conference talk in which the speaker had said that when in the car, their family would sing Primary songs to keep from arguing. I told the children we were going to take turns picking songs–any song they wanted–and sing it together.
Once we got singing (after the initial grumbling), the mood changed immediately! We sang some Primary songs, some Disney songs, whatever.
I was delighted when I had to drive somewhere else on another day, one of these children suggested we sing again. Soon we were singing and laughing again. Who knew driving together could be so much fun?