We just did a trip with our almost 3 yr old and 1 yr old twins end of May. We drove from the NW burbs to Dallas, TX. Now we did the trip on the way down mostly overnight. We left our home around 2pm (Nap time)and the drove to Springfield, IL and stopped there for dinner at my husbands father's house and for the kids to run themselves ragged. Then got everyne in their PJ's and drove the rest of the night to Dallas. For this part we just had some small books, books on tape, everyones favorite blankie/animal.
Coming back from Dallas we broke the trip up into 2 8 hour days of driving. We brought a magna doodle (big for our oldest & little for the twins), pop ons (they stack together), a few new toys they had never seen. Oh, the fridge toys...it's the alaphbet and you put the letters in the holder and it tells the letter and sound we used this along with the animal one (By leap frog) and it was a huge hit!
We did use our DVD player for a few shows but not as much as I thought we would.
Plan to stop about every 2-3 hours so they can get out and move around....3 hours was our breaking point. The biggest thing is if you can plan as much driving as you can in their nap/bed times.......you'll get more driving time in!
Good luck & have a great trip!
Jen