D.K.
Kids' music helps to keep my kids happy... we switch to adult music when they start to get sleepy. You can borrow CDs from the library. We've never done DVD players in the car, although we have played DVDs on our laptop at our destination.
I used to sit in the back with the kids while my husband drove, so that I could hand them snacks & pick up every toy that they drop... but I've found that we're all a lot happier when I stay in the front & they realize that I can't constantly hand them stuff. Otherwise, they were constantly fussing for one thing or another.
We do 4 or 5 trips to visit my husband's family each year... it usually takes us about 8 hrs, including the hour-long meal break. Honestly, our trip is usually much better than we ever expect it to be. Although maybe it helps that they're getting used to doing it.