1) Entertainment: Thank heaven for DVD and MP3 players with headphones! These have been essential for keeping peace, enabling each child to watch or listen to what they like along the road, while not disturbing everyone else. A carrying case to store DVD's is essential, too! The plastic sleeves maKe keeping up with the disks so much easier than all those movie cases that too often get stepped on and broken.
2) Activities: We tried the magnetic car games with limited success on our trips. What honestly worked the best on every ride was a pack of spiral bound notebooks and a box of pens and colored pencils. When it's back to school time and the big stores sell the spiral notebooks for ten or fifteen cents each, we stock up and buy a couple of cases. Then it's easy to grab enough notebooks for each child to have a fresh one for the journey. Some draw, some take notes about the interesting things they see on the road, some use them for tic tac toe and hangman games with a sibling. We settled on colored pencils rather than crayons because we learned what happens to crayons that get dropped and forgotten when the car gets hot inside. If you do colored pencils, make sure you bring a couple of pencil sharpeners with covers that store the shavings!
3) Food: We seldom stop along the road for food unless it's a grocery store. Fast food is pricey and most of the time the food is high in fat and sugar and low in nutrition. That kind of food makes for cranky travelers a couple of hours down the road! So we fill up the cooler before we go with ice, drinks, fruit, and snacks. And don't forget to pack some small plastic bags for trash, those are must-haves.
4) Limits: We drive no more than about 2 hours without stopping. While it makes the trip a bit longer, it makes it a whole lot nicer if we stop, get out for bathroom breaks and stretching, and get rid of any trash we've accumulated.