We also regularly travel 15+ hour trips to our parents. For the rare long trip it may not pay, but the portable DVD was a wonderful purchase after many, many tedious trips (my kids are pretty little for audiobooks). We put in a movie the adults enjoy listening to also...it helps since my DH is not much of a "car talker".

I second the Highlights vote. We also play the license plate game (see how much variety you get).

Have fun!