F.C.
C.,
While the most obvious advice would be to get a DVD player so the kids can watch movies, I suggest getting books on tape/CDs so the whole family can listen and imagine the action! This would include everyone in a discussion/conversation especially if you stopped the cassette/CD once in a while to talk about it.
Also, come up with a pose that the kids will do every time you make a stop- and then take a picture of it. I remember my sisters and I used to pose purchasing a soda from a vending machine. it's funny to see the pictures now of us and these vending machines from different cities. It's silly and fun. Your kids can also collect postcards, rocks, leaves, etc. from the different stops.
Since you've already done some roadtrips, ask your boys what they want to do.
Have fun,
F.