Hi Jordan,
I'm happy to answer this question for you because I've had to come up with things a million times over the years. The ones that worked for me the best are:
1. Playing "I spy with my little eye".
2. Snacks (cheerios, goldfish crackers, fruit snacks, etc.). A friend of mine actually strings cheerios or fruit loops on dental floss to make necklaces. She says it works like a charm. I wish I had thought of that when my kids were little.
3. I'd buy little things before our trip and not show them anything until they started getting bored. (Sticker books, little toys and books, crayons and coloring books). I was famous for going to McDonalds months before the trip to stock up on new Happy Meal toys!
4. I would also scroll through my digital camera and show them the pictures we had already taken during the trip.
Also, I don't know what time of year you'll be traveling, but if it's hot get yourself a cheap little spray bottle and fill it up with cold water. Those lines are long and hot...my kids loved it when I sprayed them to cool down.
I hope this helps! Have a wonderful time on your trip, and wish your little prince a happy birthday from me.