We're on an extended road trip and have been driving for hours with our children. All three of my children. My oldest is only four and a half and my youngest is 8 months old. While I'm no expert, I thought I would give some tips that we found worked for us while on the road for a long time.
Snacks, snacks, and more snacks. We had both boys get their little backpacks and fill them up with all the snacks they wanted. We purchased shelf stable chocolate milks, graham crackers, NutriGrain bars...anything they wanted (that was healthy of course). This helped a tremendous amount to keep them happy because they don't often get chocolate milk or NutriGrain bars. So they felt that they were having something super special and I got another hour of no crying! We usually give them dried fruit, but this time we let them pick which one they wanted and they felt grown up because they put them in their own bags and kept them by their seats.
We also used a DVD player to help them zone out. Bad mommy, I know, but it helped so much! We brought along paint with water books and let them go at it with those...my oldest son was also engrossed with his handheld gaming system.
Another great tip was to let them get out every three or four hours and stretch their legs. We'd go to the play area at McDonalds and let them run around for at least half and hour. The release of energy and boredom was awesome. We made sure to let them run around for a significant amount of time so they didn't throw a fit when it was time to get back into the car.
We've been having a blast while driving around and seeing new things. I highly recommend taking a road trip with your children. Even if it's just to somewhere close like Austin. The drive can be fun and educational. We saw windmills from a wind farm and later some oil rigs so we talked about the differences between the two and how they affect our day to day living. Not to mention that all of us were blown away (ha, ha) by how big the windmills were!