A group of my friends who travel to Dragon*Con each year (a 10+ hour drive), and they usually run a mini RPG campaign in the car. I know they have done White Wolf's Mage, Vampire, and I think they did a mini D&D campaign last year. Using the dice roller app on a smart phone makes it tons easier to not lose dice in the car.
Sadly, we won't be able to participate in this, since the car can only hold so many people, and my husband and I are driving in a separate car.
Other options include books on tape, or getting the mini magnetic board games - I have a whole host of those which can be amusing at times. Or, pick up a handicraft... Learn to knit, crochet, embroider... It can actually be really rewarding, and comes in handy for some costumes.
Personally, I'm going to be swapping between working on my 4th Doctor's scarf and making friendship bracelets to hand out while on my upcoming trip. When I'm not driving, anyway.
Party Pirate Pinkie Pie (MLP:FiM) 100%
Eris Discordia (MLP:FiM) 100%
Convention Line-Up: BronyCon 2013 - Dragon*Con 2013
Eris (Female Discord, MLP:FiM), Casual Gala Pinkie Pie (MLP:FiM)
The 10th Doctor, blue suit variation (DW), Queen Mab's Party Dress (Dresden Files: Cold Days), Party Pirate Pinke Pie