J.B.
I think the key is having lots of different things so you can switch when she loses interest in one thing. I have not taken a 12 hour trip but many many shorter trips in the car and on the plane and here were things that worked well for my son...
1. lots of picture books, especially those with tactile things to do, where you can open the flap, or touch things
2. snacks: lots of different snacks, like half-grapes, blueberries, raisins, cheerios, things your daughter can hold herself and feed herself.
3. sticker books (if she shows any interest in that...not sure about the age)
4. children's music cds, to sing along...we got through hours and hours in the car having some good music to listen to and sing with and do finger plays like itsy bitsy spider and wheels on the bus
5. my son also really liked this hand-held music "ipod" that is available at Target for around $10, I think the brand is "Parents"? he could hold it up to his ear and select the music himself
6. a mini back-pack or bag with several plastic animals, or other small favorite toys to hold and look at and play with. She can pull them out of the bag and put them back into the bag....in fact just transfering the items is a great activity in itself.
7. this little piggy is always a favorite game
8. bring a cloth or towel to play peek a boo
9. hand lotion...have her put it on herself...this will take up some time and you can do foot massage too
That's about all I can think of at the moment! Good luck!
J.