I guess car safety is one of my obsessions. I have completed EMT training, my husband is a volunteer fire fighter/EMT and I've had many close friends who have been first responders of some type. I've seen and heard so much about the injuries that can occur from having loose objects in a car, and I'm very strict. For more about missiles, read this or visit http://www.car-safety.org/ which has tips on everything related to safe driving.
Of course, the flip side to having an empty car is that mom isn't nearly as safe of a driver when kids are bored and whiny. My solution is to allow soft, lightweight toys. A big favorite for my children are cloth books like the that rear-facing Linnea is "reading" in the picture.
Amazon.com has a huge selection as does Borders. A few I really like are the Usborne cloth books like Mouse and Snail ($12.26), which have bright simple illustrations and fun textures, and the Roger Priddy cloth books like Snowy Bear and Friends and Fuzzy Bee and Friends ($8.95), which also have a variety of textures as well as crinkly parts and flaps and bits to pull. Haba-lovers might find it worthwhile to spend $15.99 on a Catherine Caterpillar or Max Mole. These books include a soft finger puppet and are available from Oompa.com. I don't have these books and they received primarily poor reviews on Oompa.
Cloth books are a hit with both Arielle (3) and Linnea (11 months - today!).
Linnea has this airplane which fastens to her strap and has soft rings for chewing. There is also a bear in a car. The airplane has gotten mixed reviews from Linnea. It is very cute and is chewable, but doesn't hold her interest as long as a book or doll.
I know, I know! It has both plastic and is MIC (as are most of the board books), but it is made by a French company with an excellent safety rating and European standards require safer plastics. In the car, I think that potential missiles and unhappy children are greater safety hazards than unpainted toys that are made in China by companies with a good reputations. If anyone has other options that are not MIC, let me know!
Arielle loves dolls. Our Nicki dolls ($19.99) more or less live in our car seats. These are made in Germany out of all natural materials. We've had Nicki dolls around since Arielle was a baby, and these 8 inch, light-weight dolls are great companions in car seats, diaper bags, restaurants, and at home. Arielle still enjoys "Baby" after three years. I love the Kathe Kruse Poupe Rose ($16.99, also available in a boyish blue/green) for babies who still drop things.
A no cost option is to bring the smaller of the numerous stuffed animals that magically accumulate around a child out to the car and store them in a cloth bag. Make sure they are machine washable (as are all my suggestions, as they will fall into puddles and get stepped on/spilled on).
And, of course there are Arielle's ever present finger puppets, but that is another post.