J.,
I recently took my kids (age 25 months and almost 4) on a 7-hour car trip by myself. I took favorite books that they could "read" to themselves. For the younger one, choose board books like Eric Carle's Very Hungry Caterpiller and make sure you read it to him many times before you go so he'll know the story for himself. The larger books got my son frustrated because he couldn't turn the pages and they were too large to fit on his lap. We also have a book with a toy piano on it--purchased at Marshalls. Car trips are where I whip out all the electronic toys that I keep put away while at home. Leap frog leap pad is ok, but again, make sure your little one knows basically how to use it. I also packed a bag with all kinds of snacks in it that they could eat whenever they wanted. It included a couple of treats like a lollipop and a few MM's as well as small portions of nutritious finger food food. When we stopped for a potty break (for my 3yo) I let them choose a small bagged snack to take back to the car (cheese puffs were chosen-ugh--but it was their choice). That lasted another 30 minutes. We sing songs--lots and lots of songs. You can make up new words to old songs. For us, "She'll Be Comin' Around the Mountain" is a favorite because we take turns adding verses--she'll be wearing pink pajamas..., Mommy will wear..., Daddy will wear..., we will all eat peanut butter an jelly or chocolate pancakes...we will all drink..., we will all play..., we will all go to sleep... the possibilities are endless. Also "Happy and You Know It" is good because you can tap, flap, wiggle or shake countless parts of your body, plus it gets them moving in their seats. CD's are good too especially if the songs are familiar to them. Oh, and don't forget the pitstops...see if you can find restaurants with a play area. If not, just let them explore inside if it's not too crowded or outside away from the parking lot. Be careful of doggie droppings outside since this is what pet-owners do too.
We also try to time our departure time carefully. For our annual Outer Banks trip which is 11 hours, we leave around 4:00 am. That way the kids can continue to sleep and we can get down the road a ways before stopping for breakfast. Be sure to pack the car the night before and write down exactly what you have to do in the "morning" before you leave since you will probably be groggy and likely to forget something. (I sound really organized, don't I--LOL--I should try to follow my own advice this year!)
Anyway, I've gone on long enough now. Happy Travelling,
Laura