My first peice of advice is stop every tow to 3 hours if she is awake. Just stretch the legs a game of chase or tag bathroom and pick up all stray toys in the car. Remeber they can not move around as easily in their seats as we can. We found that rest stops were great for this. When we did not stop we usually payed for it later and ended up taking a much longger break later. It is amazing waht a 10-15 min break can do.
A couple of little toys that she has not seen before are great. you can even wrap them if you want. They can bring a couple of hours of quiet play time.
Magnadoodles are cool. I do not know how many times we would pass it back and forth drawing pictures and erasing them. We took a travel one.
Stickers and post it notes are fun some kids like to play with them. I have done this on fieldtrips ( I worked at a school) but my son could care less. after abotu 10 seconds.
Favorite snacks in little baggies. They stay contained and then only a little is given at a time.
We like to stop at national parks wherever we are going and we alway get him a picture book. We like these because they can grow with him.
We like rocks and little things like watches, hair clips, and things like that in the car too. he loves playing with them
In the last two years we have driven to CT, CO and TX all on different trips from CA. He love to explore make it a game and talk about waht you are seeing. Educational and fun.