I think that the suggestions so far have been great...a have a couple more.
First is a jug of water...not for drinking but just in case the radiator acts up. I have passed many a car on the highway when I drive that is overheating.
Also a can of fix a flat...it can help you get to the next exit usually if something happens (this is a must for me since I got a flat on my last trip up to MA from FL).
Flashlight has been mentioned but also if you plan on reading or have kids that will be reading in the car, a book light is a must after dark.
A cell phone. If you don't own one, you might want to look into getting one for the long drive. There are lots of states where you have many miles between exits.
If you don't have a cell phone, paper and markers in the car so you can write a note to put in the window asking people to call for help.
If you are coming down any time prior to April, I would suggest that you keep a small, compact snow shovel and some de-icer in the car in case of bad weather (blankets are also useful just in case).
A lot of my suggestions come from driving up to MA from FL every Dec. for the holidays.