Last summer we drove across Minnesota and South Dakota and half way across Wyoming with a 3 year old and a 5 year old.
Look for travel bingo games. They have little windows to close so no pieces to lose. You have to find different things out the window (barn, flag, fire engine, school, etc.). My kids loved it.
Books on CD.
Pipe cleaners! My kids love making things out of them and you don't need any other supplies. Just be careful because with some of them the wire can stick out of the end and be sharp. Wiki sticks are fun too but can get extra sticky if left in a hot car.
"Magic" invisible ink books were a big hit. Color with a clear marker and it reveals color or a hidden picture. The scratch art books were fun too.
Last year I found bubble containers that were non-spill. Very fun to hold the wands up to the air vents and watch the bubbles.
We have some magnetic play boards that are similar to a sticker book. Or take a cookie sheet and before hand cut out paper dolls or pictures from magazines that can be used to create scenes or make silly faces. You can also make puzzles. Glue magnets on the back and they will stick to the cookie sheets.
Lap boards were a great thing to have. It made it easier to color and do things like that. The cookie sheet would also work. When I was a kid my mom made us lap boards. Basically it was a cookie sheet with a lap sized beanbag glued to the bottom of the pan. The edges of the cookie sheet kept crayons and toys from rolling off.
I had a bag of goodies that I packed. Whenever we stopped or whenever they were getting bored I'd pull out something new. I also had a set amount of cash that when we stopped the kids got a couple of dollars to buy a little treat. Sometimes it was just a tattoo from a gum ball machine or a post card.
Your 6 year old might like having a map to follow your trip on. Pick up a highway map and highlight your route.
They also liked playing on the iPad for short times.
Best thing we did was stopping for sit down meals (and eating outside if possible). It took more time, but the kids got to be out of the car for longer. Or pick up Subway and eat at a park. I was actually surprised just how long my kids were content to ride.
Have a great trip!!