I just finished driving from MN to New Brunswick with just me and my 5yo son (4200 miles). I thought about buying a DVD player and decided not. I am glad I didn't as we had a great trip without it.
Things I did:
1) Books on tape/CD from the library. We went together and checked them out. We got a few that were shorter and had a book to look at and other longer chapter books without a visual. A good one for your ages is Frog and Toad. It is a whole bunch of the Frog and Toad stories and my son was addicted to it. Other good stories included A Castle in the Attic and The battle for the Castle, both stories by Elizabeth Winthrop. If you want to buy some kid's books on tape my favorite place is Barefoot Books. http://www.barefoot-books.com/us/site/pages/home.php We have a couple from there and my son really enjoys them.
2) Pack their own snack bag that they can eat out of whenever they want. I pack Goldfish or pretzel, trail mix, individual fruit cup, a half sandwich, one juice box, some dried fruit, individually wrapped Fig Newtons, a spoon, and a cloth napkin. I put it all in one of his fun lunch boxes/bags. I also keep a small cooler with string cheese, carrot sticks, grapes.
3) Plan one longer stop each day that is a fun sightseeing place and allows them to stretch their legs. It doesn't always have to be a Playspace at a fast food restaurant. Research small Children Museums or other attractions on your route. Or a historical site. On our drive out we stopped at the I Love Toy Trains Store, a small town Childrens Museum, Niagara Falls, and the Saratoga Battlefield. On the way back we stopped at a bigger Children's Museum, took a ferry ride, visited the Saratoga Battlefield again and Niagara Falls again. Relax and let them have 1.5-2 hours of time to run around and play. You can always drive a little longer at night when they are asleep.
4) I bought some inexpensive new toys and wrapped them. He got to have one mid-morning and one mid-afternoon. A new little stuffed animal, a sticker book, a new Hotwheel, a coloring book, or a picture book are some ideas.
5) Music CDs...we sang along together.
6) Play Twenty Questions (my son is really good at stumping me because he picks things like air or coal...lol). Other car games might include a letter search, I Spy, color search, travel bingo.
7) Just chat about stuff you see out the window. My son got to practice his numbers by reading the speed limit signs for. He asked questions about things he saw out the window. It was great.
8) Your 6yo is old enough to start learning to follow your route on a map or in an atlas. Highlight your route and have him look for signs that match the towns on your route. By 7yo I was helping my mom navigate through Chicago...lol.
9) For cleanliness, all garbage went into a plastic bag and was tossed at each gas stop. When we stopped some where and my son got out we always did a quick toy cleanup and tossed loose ones back into his toy bag. I also packed a separate overnight bag that we could grab to take into the hotel rather than unpacking or digging into our main trip clothes. I included pajamas, change of clothes, toothbrush, comb.
Have a great trip and make some memories along the way.