I'm with JGM on this one. We minimize projectiles as much as we reasonably can. The stroller is bungeed to a cargo hook in the back of the van, diaper bag goes on the floor in the front seat or behind my seat because I often need to access it during outings. On longer trips, the kids are allowed water in a sippy (DS) or sport bottle (DD). Gear is packed so that the heaviest items are on the bottom (pack-n-play), lighter items on top (pillows, blankets, etc.) We have one soft toy in the car for DS, otherwise there are no other loose items/toys... oh wait we do have a tissue box on the floor between the front seats, wedged in by the console. We're actually driving from MI to OK in April and I've been thinking about toys/activities for the car. I've made the parental decision to allow DD to use her Leapster during this trip, along with some other small toys such as a travel size Doodle Pro, Aqua Doodle, etc.
Really, I'm all for reducing risk as much as we can. However, sometimes it's just not reasonable for us... no amount of soft toys will satisfy my kids on a 14 hour drive, so for that particular occasion, we'll make an exception to the amount of potential projectiles allowed in the vehicle. Kind of have to weigh the risks... possible projectiles vs. screaming children for 14 hours.