Having just done the NY-Florida thing last month, some of the things that worked really well for us was investing in an adapter for my laptop (played dvds and had a bigger screen than a car dvd player). This additionally allowed me to upload my pictures each night and I didn't have to buy extra memory cards. Good thing because I took over 1500 pictures!
Another thing that was HUGE was something my mom bought the kids, it was coloring books (Disney themed) that had something called bathtub stickers in them. They were like little window clings in disney characters that they used to decorate the windows of the car, they had hours of fun with that. IN fact, I still have nemo and Dory on my car.
My 4 year old LOVED the gameboy and games that Daddy gave him just before the trip. I think he played that for 3 hours a day, the hard part though is that he's not allowed to play it like that now that we are home.
Fun little snacks that they aren't normally allowed to eat much of were great treats. We stopped at each states "welcome center" and played tag to run off steam and it only took about 5 minutes and was a nice stretch for mom and dad too!