I suggest bringing a few wrapped surprises. Just don't let her know you have a surprise because, if she is like my dd that is all she will talk about is what the surprise is. We picked up a few coloring/activity books with brand new crayons, a my little pony (travel side), a journal to write/draw about her journey/vacations. Our local family dollar or Ocean State Job Lot or Big Lots is usually good for having reasonable priced trinkets. Also a dvd player has been a blessing. We also play ride games like.... we begin with the letter A and sing the old "A my name is Alice, my husbands name is Alvin, we live in Alabama and we eat apples" game. We however found this to be too easy so we changed it to you have to use disney names. Even our adult children have fun playing this. We also play what disney character am I. We start with no clues, then begin to give clues until someone guesses it. You would be surprised how much time this can take. We also play license plate games. When traveling as a kid it used to be license plate cribbage, however most license plates don't have enough #'s to use just one plate anymore, and 6 maybe a little young for this. So, what about find a license plate that the numbers add up to say 11 for instance.
Pack snacks, but remember beverage + food = more rest stops. We also play seek and find. ex: Who can find a wishing well, or a cow, or a black truck. Be creative, my son once used who can find a ram. We thought he was just using this to stop the game when he was actually waiting for us to notice Dodge Truck's hood ornament the Ram.
Have fun. I'm sure you will get plenty wonderful ideas on this thread.
Also, does she have certain music or movie that soothes her? We used to show our kids
Disneyland Fun sing a long before bed. The last song is when you wish upon a star. To this day (about 17 years later) they will begin to yawn at the end of that video. Honestly, it doesn't work as well for my dd7, but we keep trying.
Oh, just remembered.... not sure if your dd is used to long rides, but just in case bring a bag in case she gets motion sick. One of my ds used to get sick if he drank orange juice while riding in a car. (took us awhile to figure out it was the OJ. Never had a problem when we stopped letting him drink that while riding) Just a thought.