I may be an odd-ball here, but I generally don't let my kids eat anything that can't be easily vaccumed up (crackers, goldfish, etc), nor do I let them drink anything other than water while riding in the vehicles. These rules of course, don't apply to me (dew or coke all day).
I do this for two reasons:
1.) The mess that they will make
2.) I don't want to have to stop the vehicle in an instant because they are choking on a sucker.
3.) Biggest of them all: I can't stand the smell of old food especially when its hot out, that is forever entrapped into the seats, carpet, & headliner.
Other than that, anything goes, much to my wife's disapproval, i.e. standing on the seats to pop out of the sunroof.
I've been lucky that all the barf-a-rama's in the car have been contained to their carseats!
I figure once the kids are older, i'll re-do the interior anyway, so let it wear as it is.