I recently cleaned out the back of the minivan after our trip to the Wisconsin Dells. It took me hours. It was completely and totally disgusting. I vacuumed, cleaned carpet, wiped everything down and took absolutely everything out of the car.
In the past, each kid had a bag of stuff to play with next to their seat. That bag grew to a bag and a plastic tote and anything you could shove in the cup holders. It sounds like a good idea to have things on hand, but the baby get frustrated that she can't really access the bag, and she tends to drop her 5 snacks all over the floor.
Now that car is clean, today, I decided to bring her 3 books. And Tate could have 3 things. And at the end of the car ride, those 3 things (each) come back into the house. I'll tell you why this is great--the car stays clean AND they actually play/read with those 3 things. There isn't a mound of stuff that they throw on the floor, and they are happier. I had read once that if you give kids too many choices, it overwhelms them. This seems to be the case in my house. (Yesterday, I cleared out Soleil's little "shop", and had the same surprising result--she played with the 10% toys I left behind!)