One of the most annoying thing for me about road trips, is that the activities and toys I have to keep the kids busy seem to be all over the car. When the sliding door opens at the gas station, for us all to rush in and use the restroom, coloring books, water bottles, toys, shoes, pillows, it all comes spilling out. It's especially frustrating during winter weather, or if the person before you using the pump decided to dump their soda all over ground- and now all your stuff is wet, sticky and dirty.
We solved this by making these car organizers. It' s just a large rectangle of fabric with pockets sewn on, and ribbons for tying it onto the seats. Each child had their own, they could put what they wanted in it- but once it was full- that was it for what they could bring.
Another great way to do this with out the organizer, is to put those karabiner clips on the head rests and hook back packs on to each clip. That worked well on a trip when we were headed to all different locations and the kids wanted to bring their stuff in at night.