I also let me daughter (2 1/2 years old) eat in the car and we do have toys all over the place. What I have just started doing, as another mother suggested, is take a target, walmart whatever cloth bag and pile all the books, toys etc. in them. Every time we get out of the car I quickly throw all my daughters stuff in the bag and the car looks "tidy" and neat...
For us it has been more of the snow and salt that has gotten everywhere, we just got a car organizer for the back of the seat she sits behind so if it gets really dirty/gross I just take it out and throw it in the wash. We also have a seat protector under my daughters car seat which helps with spills...
If you really want to stop eating in the car that would keep it cleaner but I know that I tend to be very busy with work and taking my daughter places we eat in the car too. So if you must eat try to eat foods that are east to clean up with a hand vacuum. Stay away from sticky foods, we do eat grenoal bars which cause a big mess but for the most part easy to clean. All drinks in a sippy cup in the car to advoid any drink spills because that is really messy. I also stay away from milk or straight juice, they can make the car smelly or sticky if not cleaned up ASAP... we do water down juice or just water.
So that is what we have done... I hope that you find something that fits your life!