We do long drives (12 hours in 1 day to upstate NY) all the time with our 3 young children (6 months to 5 years). The best thing for us is to have a long stop every 3-4 hours. Most of our stops are under 10 minutes - long enough to use the bathroom, change a diaper, fill up with gas and grab food to eat while driving. However, we use the AAA triptik to find the rest areas and plan for a 30-60 minute stop. We bring a soccer ball, head to the grassy area and make the kids run around and play for 30-45 minutes. We get them really tired, then get back into the car and (hopefully!) they all fall asleep for awhile. :-)
You might want to try rigging up the portable DVD player so it hangs in her view. I wouldn't let her hold it because she's so young (at least, I wouldn't want my kiddos to hold it at that age - they would've dropped/broken it).
Hit up the dollar store for a bunch of small new toys. Or put away some toys at home so they are "new" for the trip. Small board books are good and easy for little hands. A small magna doodle is good. So is a stuffed animal/doll with buttons, snaps, laces, etc. Some soft fabric books have lots of buttons, snaps, laces, etc to keep hands busy (my kids all loved My Quiet Book). Use plastic links to 'leash' toys like stuffed animals to her carseat so they don't fall to the floor.