I'd like to start out by confessing that we own a dual-screen portable dvd player for entertaining the kiddos on road trips. That said, I do not often allow it's use for trips to VT. In my mind, a short(ish) 4.5hr trip to VT does not require entertainment of the movie/television variety (though I HAVE resorted to using them for the trek to VT). We have used the dvd player primarily for longer trips, like when we've visited Grandpa and Grandma Pauline in KY and when we went to PA to visit niece/cousin Allison, her hubby, and their daughter.
For our more-frequent trips to VT, I try to keep the kiddos entertained in other ways. I have created scavenger hunts (typed and printed a copy for each child) -- with normal things like "UPS truck" to not-so-normal sitings like "elephant". I have also typed and printed out sheets that list every state in the US so the kids can keep track of all the different license plates they see. The kids all know how to play the alphabet game.
Most recently (just in time for our Thanksgiving trip), I taught the kids the "Padiddle" game. This game is for when it is dark outside. You look at the headlights of the oncoming vehicles. A vehicle that has one blown headlight is known as a "Padiddle". We counted how many "Padiddles" we spotted. If memory serves correctly, there were less than 40 that night.
I remember playing this game when I was about the age of my big girls. It was a game that Mom and Dad introduced to us kids. Probably for the same reason I introduced it to my kids -- to give them something to concentrate on besides annoying each other!
I told the kids that we used to try to see who could spot the "Padiddle" first, and then we counted them. Eventually we stopped yelling, "Padiddle" and simply counted them aloud. Everyone in the car always knew what Dad meant when he loudly stated a number.