D.T.
We do a 10-13 hr drive several times a year with 3 young kids. It's not nearly as bad as most people assume. Kids don't need constant entertainment - let 'em stare out the window daydreaming for awhile. We look forward to the long drives and know it's a full day of fun we spend together as a family.
My kids love car bingo. You can buy an all-in-one cardboard board with little sliding red windows that cover the squares - no loose pieces. Even my soon-to-be-2-year-old likes playing with it. She doesn't play bingo, of course, but she loves playing with the little windows. We got ours at the teacher supply store in Castleton.
http://www.onestepahead.com/catalog/product.jsp?productId... -- we got one for each of the older kids and will soon get one for the youngest. It's a good way to contain coloring books and gives 'em a surface to draw on if they want. However, my kids rarely use it as a desk and prefer to just put the coloring books on their laps. We've had one for 5 years so they are sturdy and hold up well. One Step Ahead also has tray/table type things that sit on the lap. Check out their travel section for lots of neat ideas.
Sticker books, Color Wonder markers/books (crayons melt in the heat when left in car, regular markers are messy), colorforms (they stick to windows!), lacing cards, wooden beads with string, small box/bag of small plastic animals (some might be choking hazard but just keep an eye on the little one... last week our 1 year old spent hours playing with the animals during our all day drive), and of course, portable DVD player (with young kids it's best to hang from seatback so they dont' ruin it). We limit DVD viewing to just a couple hours during the all day drive... about an hour before lunch and then again once it gets dark out (and hard to see to play with toys).
Take a soccer ball and/or frisbee. Sometime in the early afternoon, we'll stop at a rest area for 30-45 minutes to let them run around in the grass burning off energy. They are all ready for a nap after that.
I found lots of neat toys at a teacher supply store (United Art & Education in Castleton) - lacing cards, bingo cards, colorforms, sticker books, slinkies, beading toys, small felt boards, chalk board books, etc.