Almost half of us (47%) carry sunglasses in our glove box, but what about other weather conditions? I've called this item "Weather Protection" to help you think about carrying items that might help if there's a sudden change in the weather. Alternate what you carry as the seasons (and space) dictate. Rainy season? Totes, Samsonite, Rainkist, and Leighton all make micro umbrellas. Want to save even more space? Just throw an emergency poncho in the glovebox! In the winter, take out the raingear and toss in a spare cap & gloves. Land's End makes a Thermacheck 100 Fleece cap and gloves that are amazingly warm for being so thin and light.
I covered vehicle title in item #1. Don't carry it unless you have to. keep an old plastic film container with some dollar coins and some quarters instead, for all the little things that only cost a buck or two. Toll booths, pay phones (are there any left?), snacks at fast foods drive-ups; there are plenty of times when a little extra change can come in handy.
Ever heard of Adventure Medical? They've put together all kinds of well designed first-aid kits for lots of uses. Check out their Pocket Medic. It's perfect for throwing in a glove box.
A snack bar might keep you going, but a digital tire pressure gauge might just keep your car going. It's a little, inexpensive item that can make a big difference. Every time you fill up, you can check your tire pressure. Why? When tires are inflated properly, you get better fuel economy, better handling, shorter stopping distances, longer tire life and your breath smells better. Just kidding on the last one.
Vehicle operation; maintenance; entertainment systems; navigation - today's cars are complicated. Your owner's manual is your bible, dictionary and encyclopedia for everything about your car. It won't do you any good when you're on a trip and it's at home in the filing cabinet. You don't really want to put 3 year old ketchup on those hot, fresh fries anyway - do you?