We've recently found that our 2 year old loves a particular kids's CD. It has songs that have motions like the Hokey Pokey, Where is Thumbkin, The Wheels on the Bus and it keeps her pretty engaged. The hardest part is finding a good time to transition to NOT listening to the CD for the parent's sanity! But she won't usually complain when I turn the volume almost all the way down after she's listened to it once or twice through.:)

Amazon has a lot of kid's CD's and allows you to preview some of the songs.

We bought one of those magnetic drawing boards for our 2 year old son, and that will often keep him entertained for an hour or more at a time.

What about a portable DVD player? Watching her favorite movie could help for an hour or so! Or what about a game where she has to find things outside. Another cool thing are those leapfrog books? We listen to the Little People CD's every time we are in the car. My kids love singing along with the little kids.

When doing long drives with the kids we always kept them engaged by doing the corny sing a long thing as well as eye-spy. Keeping them engaged with what is going on outside the car by asking them questions and prompting them to make up their own stories. Also stoping every couple of hours during an 8 hour drive burnt off the excess energy and stopped me from getting road burn.

Didn't have videos at the time and besides seeing things along the route is part of the trip!

We have traveled a lot with our almost 2 year old and we just make sure we have a variety of things to entertain him. Our new favorite is the portable DVD player with some Elmo videos. This can keep him occupied for at least 45 mins and then we have to switch it up a bit. Try to leave around her nap time so that even if she doesn't fall asleep right at her nap time, hopefully she will get tired enough to at least sleep for a little bit. Here are some other things we bring with us on longer trips, a lot mentioned above: books (sometimes a new one), magna doodle, snacks (a special treat goes a long ways), smaller toys, kids cds, etc. Be creative and I think if you add "new" toys to the mix she will be exited about that. Also one thing we have found helpful is to make at least a few stops along the way so that our son can get out and run, I know it isn't ideal, but a little bit of fresh air is good for them!

What about a laptop? You can get one fairly cheap and lightweight. I've seen one from "One Laptop Per Child" that is even waterproof and is made specifically for children. The battery life is several hours.

No one has mentioned books? My older child loves the little people books. He can't read them but they have tons of flaps to keep him busy and come in a variety of topics. We also have some children music CDs (They Might Be Giants have 3 children's albums about the alphabet, numbers and science that aren't as annoying as most children music CDs). We never had much luck with our portable DVD player in car so we never use it anymore.

We had similar problems until we were encouraged to place the child seat in the middle of the back seat.

www.childcarseats.org.uk/choosing/position3.htm

Then instantly her car mode changed. She was more calm, relaxed and observant. We realised that the view from the middle back was the major factor. She could see us both and more important look out ahead.

I am not sure where your child-seat is placed or whether regulations in your state prohibit you from placing the child seat in the middle of the back seat, but if you can you should give it a try.

Another point is we always talked with her, so she did not feel left out or alone at the back of the car.

Why not sit in the backseat with your kid, and do some sing along games? Adults can choose to sit front or back, kids can't (at least not here in Germany)

Story CDs can be good as they get a bit older, we like Winnie the Pooh read by Alan Bennett, it doesn't even drive adults crazy.

Does she get car sick? Mine tended to as they got around the three year old age range, so books and other in car activities tended to be a bad idea.

If she can't manage eye-spy yet, how about "first one to see..."?