We took a trip to Disney (from PA) with our almost 3 year old and it was a long 3 day car ride. Surprising to me he did pretty good. We did cheat a bit and have a small DVD player that did help pass the time but we also sang a lot of songs, looked for cows and horses when going through farm land. Counted trucks and busses when going through the city's. If you have two adults traveling it does help. I brought his mini etch-a -sketch (not sure on that spelling) but we would draw simple things like cars, boats, flowers, ladybugs, turtles ect. I did read some books and tell stories. We did a fair amount of talking about the trip to Disney itself to prepare him for what he was going to see and do on his vacation.
One very important thing I learned was the the car seats cause your child to have a fair amount of flextion at the hips which caused my son's legs to go to sleep over time. He complained about his legs hurting him and it seemed that he just had some numbness due to lack of circulation. Once I learned this I just took a small childs blanket and folded it under his hips to raise him up some. This made his upper legs more level and gave him some relief. You figure they can't move around and adjust like we can with those safety seats. I still felt he was safe because of the 5 point harness. He just looked a lot more comfortable with his hips raised up a L.. This of course with getting out to run around on breaks was great for us. Even just 10 or 15 min. of moving outside the car. Having snacks and napping "of course normal toddler routine" helped pass time also. Good luck, 8 hours will go by pretty fast if you can make it fun :)