My parents used to have us plan the route (with their help). Our job was to find all of the points of interest along the way and do some research to have details about the sites ready to discuss as we got close. Some we would stop at, others we would just talk about as we went by (for example the Cadillac Ranch in Texas, you could see it from the highway).
I actually learned to like to read while on trips with my family. With my weekly allowance, I would buy comic books and read them as we travelled down the road. Eventually the comic books gave way to paperback books.
One of our favorite games, my mom would make a list of things to look for...barb wire fence, water tower, Barbersol Signs, cows, firetruck...etc. Each of us had a list.
The goal was to find as many of the items on the list as possible. It was a good way to keep us quiet because you didn't divulge that you had spotted one of the items.