A few ideas:
1. Cracker Barrell Books-On-Audio-You pay anywhere from 25-35.00 for a BOA. When you return it to ANY CB, you will get back all but 3.50-4.00 (I think for a 7-day period). Basically, if you get it and like it, you can keep it cause you bought it. If you just want to listen to it and return it, you are renting it for a bit of change. Some of our surprises we found were Hoot, and Peter And the Starcatchers (written by Dave Barry and Ridley Pearson).
2. A laptop computer and an inverter OR a portable DVD player, headphones and your DVD collection-we let our kiddos watch ONE movie, then they have to find something else to keep their hands busy for a couple of hours.
3. New books-a new book in a series, a new author, etc
4. A bag of Disney paraphenalia...a new disney pen, a disney coloring book w/ new colored pencils (won't melt in a hot car like crayons), a disney puzzle book, a deck of Disney cards, etc.
When our kiddos were younger (they are a few yrs older than yours), we allowed them some talking time, some quiet-look-out-the-window time, reading time, close-your-eyes time, and the ALWAYS important...stretch your legs time. We traveled by car extensively when they were little (anywhere from 6 to 16 hrs at a time), and we got into a good routine!
Happy Traveling!
I just re-read your post, and realized you were asking for GAMES, not ideas!

One of our fav games is Punch Bug. We also play "If I was Going to the Fair I would take A/An" then you start at the beginning of the alphabet, going through until you get tired. Different variations....we have named states, animals, locations, etc. in alphabetical order while taking turns. HTH