Here's a fun collaborative adaption of one of my favourite activities, people watching. It's a good way to keep everyone entertained in traffic.
Choose a car or vehicle around you.
Give the vehicle a name and personality and describe it to your car mates. Where do you think it is going and where did it come from?
Encourage every one to be as imaginative as possible and use things they can see (such as a bump to the car, a car sticker etc) to expand their stories.