We never use a DVD player, in fact, we ordered our vehicle without one and everyone thinks we are crazy, but we are closer for it AND boredom IS good for kids. Being stimulated all of the time does not allow them to be comfortable with down-time.
We take TONS of road trips with our kids and did an RV trip along the Oregon Coast. Everyone survived, even our 3 month old, at the time.
We look at license plates and use the numbers and whoever adds them up first, gets a point. This is a game that doesn't last very long, so set it for 10 minutes the first go around.
Count motorcycles, then only blue big rigs, then VW bugs, or if there is something they might see along the way for your trip specifically, then count those.
Use the alphabet....start at A and the whoever sees something that starts with A first, then the next thing they see that starts with B. No points here, but just for fun.
I would rather spend money on crafts in the car rather than walkie talkies. Stop every 2 hours for 10-15 minutes and MAKE them play tag, etc. Tell them that when you stop, you need to tell someone in the other car, the coolest thing you saw on this last leg of the trip. It'll keep them watching.
Keep the snacks for the car, not the stops. Keep the stops for running and bathroom breaks.
Sit next to your nephew and keep him entertained. Keep his belly full....lots of snacks and water. Keep his diaper clean. And put more padding in those hard car seats. Honestly, we put cloth diapers between the plastic and the "padding" - and it changed the ride for my son....AND us.
There is a book called Carschooling ideas and it's FULL of stuff, so if you can find it locally, I would get it today and find what you need for car crafts.
Carschooling: Over 350 Entertaining Games & Activities to Turn Travel Time into Learning Time - For Kids Ages 4 to 17 [Paperback]
By:Diane Flynn Keith
Good luck!