If your kids are old enough to write or even color, there are lots of fun printable travel pages out there. Mom's Minivan has lots of great pages, from coloring sheets to word trip bingo. Print out a few for each child, according to age, throw them in a binder or folder and you have a ready made activity book.
When Adventures of a DIY Mom took a road trip to Disneyland, she put together activity bags for each of her children including snacks, coloring books, crayons and some travel games. She created this checker board from felt! Cheap, easy and the checkers don't slid off the board when you go around a corner.
These are a couple of marble mazes from Serving Pink Lemonade. Even if you are a novice sewer, these pouches are simply straight lines with a marble inside. The kids will have a blast rolling the marble through the aisles to get to the end.
Whether you are flying or driving with kids, there are lots of ideas out there for you. Take these or make your own, and enjoy your holiday travels!