I stumbled across a tutorial to make LED "throwies" and thought that these would be great to stick to everyone's cars for group road trips. When I was a Girl Scout, and then later when I was manager of the soccer team, we often took trips with multiple cars in a caravan. Booster flags, etc work great during the day, but they're not as visible after dark. These are simple, inexpensive, and could be made by the kids in the troupe or on the team and sold to parents as a fundraising project.
Related: More road trip hacks