If you can leave close to bedtime, that will be best. If not, plan to stop frequently to stretch and run around. We've had to do both. The night time drives were easy because our daughter slept most of the way. The day drives were tough.
We made sure we had snacks, juice/water/milk and stopped frequently to stretch, play outside for a few minutes (most rest stops will have an area to play or run around) and provided her with entertainment in the form of toys, coloring books, magnadoodle, and some interactive toys. We did not use a DVD player and I don't think the expense is necessary, at any age. I also would sit back with her for some of the drive so that I could engage her in play activities. We also brought along a few music CDs that she likes and that helped.
When she would get fussy we would try to calm her and if that didn't work, we'd stop and let her run it out. We also made all of our food stops actual stops instead of drive-thru and eating in the car. It took us a couple more hours to get to our destination, but we did so without killing ourselves doing it.
Take care and good luck! :)