So glad I came over to respond. You have received some great suggestions that I hadn't thought of and can use.
Here are my two cents - Definitely get the Color Wonder markers and paper. They are a bit pricey, but they don't melt in the heat and they can't mark up your vehicle.
I always sit in the back with my granddaughter, behind the passenger seat, that way I can move the empty seat as far forward as possible and have plenty of leg room and my hubby can see her in the rear view mirror without having to turn away from the road.
Our 5 year old granddaughter is still in a car seat. She likes it much better than a booster. She can see down the road better, if more comfortable when she is sleeping and of course is safer.
When we stop to eat, we usually stop somewhere with a playground and let her play to her hearts content and then let her eat in the car. This has worked great since we started traveling with her when she was a year old. Of course she doesn't normally get to eat in the vehicle so it is a treat to her.
Finger puppets! She has the best imagination and puts on amazing shows that I video to share with her mom when we get home.
We try to travel on her schedule. If she needs to stop we stop. If she is sleeping, we go until she wakes up. This is easy for us traveling with just one child, but watch the gas gauge. We have almost been in trouble a few times by not stopping until she is awake! LOL
People look at me funny on this last one - we travel with her training potty, wipes, and something to seal the waste up in until we get to a waste basket. Restrooms are not always available when a child needs to go and this has come in VERY handy on many occasions. I have a little basket in the back next to it with all the necessary items we might need if she has to go.
Have fun on your trip. These are precious times with your little one.
God Bless