A trick my parents would use to stop me and my siblings from complaining on road trips was something called the nickel game. You started with $1.50 and every time you complained a nickel would be taken away. At the end of the road trip, you would use your money to buy candy at a gas station :)
When I was younger, we had to take road trips down south to see my dad who was in the military. As a 2-8 year old (Traveled through those ages) I got very bored in the car. My mom was always so nice to my brother and I and did all of these. I cannot tell you how many coloring books I went through, how many times my poor mom had to listen to us sing Veggie Tales songs over and over and how many stuffed animals I collected. Very helpful. Will remember these and all the good times I had when ever I have children.
The geography game is always a good one too! You say a place, river, continent etc and the letter that ends with the next person must say a place that starts with that letter. Ex person 1 says Canada so person 2 would say America.