We just got back from Driving from Ontario to Manitioba through the states with DS ( 11 months) its a 20 hour drive. We stopped every 2-3 hours and took him out of his seat, let him crawl around and change him ect. He doesnt watch tv so we didnt do the DVD player, however next time when he's older that might be good for him and the passenger.. Rural North Dakota can get a tad bit boring.
What I did was I took away his favourite toys about 2 weeks before we left and packed them to come with us. As we drove along we pulled out his old favourites and it was like he had brand new toys, he was so happy to see his old fav's. I also had lots of snacks ready for him ( cherrio's ect) and I had 2 sippy cups of water always on the go, because well, he would drop one and no one could reach it ect.
We have a sedan so we put him in the middle seat in the back so it was easier for him to see us then the back of a chair ( he's forward facing now) which was good. He did sleep a lot but really was good, we kept him on his schedule of meals and he kept his regular naps up. But we also stopped at a hotel on our way too. Once we got into the room, I let him go nuts, he crawled everywhere and was SO happy not to be in the car seat. We also made sure that he had a bath at night too, just to get the last bit of energy out.
The one thing that annoyed DH and I was we would sometimes stop at resturants ( arby's and Wendy's ) to eat and change him and us too. They didnt have change stations! So annoying..