My phone, car charger, lots of road trip snacky food, lots of music in various genres...
My phone is kind of an all-in-one thing though.. 'Cuz it connects to the car via Blue Tooth, if Parker is with us, she can watch tv shows or movies on Netflix or something on it. Or watch something on the laptop if I'm using my phone. Otherwise, I have a playlist just for her on my 80GB iPod classic. I need to put movies on it, though.. I have a ton of space still.