Finally I couldn't take it any more. I couldn't stand the thought of those two precious little brains getting mushier and mushier with each passing mile. Before our next trip I created a car-ride scavenger hunt in Spanish. I let the kids know that there would be no video games in the car until they found all the items on their scavenger hunt. After that they could play for an hour at a time with hour breaks in between for reading, activity books, flash cards, etc. There were still loud and angry protests, but they came before we got in the car since I gave the boys plenty of advanced notice. This is a move I highly recommend--at your own house you can close the door on loud and angry protests until they quiet down and eventually cease to exist.
The scavenger hunt turned out to be lots of fun for all of us. It got the boys (and mom and dad) looking out the windows and took almost two hours. Here is a link to the document. Feel free to modify it to suit the area in which you will be traveling.