My husband can't stand the fact that my car is a trash receptacle on wheels. One time he asked me why I couldn't just clean the stuff out of the car when I got home. I walked him through a typical day: Leave at 8:30 to get to Bible study; stop at Starbucks for nourishment. After Bible study, I have to do errands, so we go to the store and fill the back of the SUV with bags of stuff that I have purchased so that the family can eat, wipe or play. We have to stop for lunch, because it's nearing toddler naptime, and I don't want to risk said toddler falling asleep on the way home and waking up hungry twenty minutes later. I get home and transfer sleeping toddler from car to crib. Then I make several trips to and from the car to get the stuff out. I'm worn out, but I still have to put everything away. Getting back to the car doesn't happen, nor does clearing out the coffee cups and cracker crumbs and sippy cups that end up in the car at the end of a long day, and multiply times five by the end of a long week. He doesn't really buy it. It's not an excuse, but it's the truth.
I have figured out one way to keep things somewhat organized. In addition to the foodstuffs, in order for cartime to be happy time (for mom and kids), we have to have plenty of diversions: board books, magna doodles, cars and trains. My husband is all for these since they tend to keep the kid chaos level down since they are entertained, but they can get scattered. My very simple way to deal with this is to keep a container in the floorboard of the car.
Pictured in the container is the very best diversion for the two to five set. We received My First Leap Pad for my son's second birthday. He was too young for it at first, but we pulled it out about 2 months after he turned two when we took a car trip. He figured out how to work it on his own and it's been smooth sailing ever since then. Each cartridge has several games and he has his favorites, in addition to listening to some of the songs, which are a little annoying, but better than whining and crying.
This leads me to my final tip: Keep certain toys (like this one) for car use only. My son is almost three now and playing with his leap pad keeps him happy on all our car outings--long and short (one thing I like about the My First Leap Pad, in addition to the fact that I think that the games are more educational, is that the books are smaller than the Leap Pad and fit easily along with the cartridges in a gallon size ziplock bag). Keeping it only in the car has kept up his interest. Before a long trip, we also buy a new book and cartridge (which a nearby children's resale shop often have used for half price) which also guarantees a little more interest.