I put together a big craft bag to have within arms reach on the road trip. It includes all the basics: coloring books, crayons, markers (Color Wonder), pipe cleaners, tape, kid scissors, glue sticks, and stickers, lots of stickers.
Post Road Trip Update: This was a great idea, especially when it came to Mercy. She loved it. I eventually just sat it on the seat next to her and let her have at it (minus the scissors because we aren't quite at that trust level when it comes to scissors, I've never recovered from the time I caught her quietly snipping her hair).