E.J.
WE are planning a road trip ourselves. So far my plan is: Get them each a roll of quarters. They keep them all to buy a gift when we get there if they don't constantly ask 'are we there yet' or fight with each other. Each #2 warning costs them 25 cents.
I am making them workbooks: Each page has an activity of colors, shapes, cars, numbers, letters, etc. I am tying to break it up so that they have a different activity per page and that this can be stretched out. Then they get a fun sticker when they identify the object. Make them 'pretend' maps to help drive, and they can color the pictures they see on the road. Have them "make" souvenier (sp?) bags and they can take something from each stop (rock, pamphlet,etc) and put it in their specially designed bag. Make them a (easy)scrap book of all the sites on the way there and the destination and let them plan a trip. We're going to cut out hands, color them and stick them to popsicle sticks to wave to other kids, trucks, animals on the road. Grab some construction paper and have them draw their favorite foods, colors, pets, shows, books, people, etc (the best they can). Burn them a CD of their favorite songs. I did the CARS sound track the it kept the kids entertained for 45m b/c they were so curious to hear the music without the movie. I burned one with themes from their favorite shows and it gets them thinking about their favorite characters! IF you like Barenaked Ladies you will love SNACKTIME songs CD. It even entertains my husband! If they can reach get an empty canister and have them put playing cards in the slot. I do this at doctor appointments and it cracks them up!
See if your library has UNPLUGGED PLAY by Debbie Conner. I lists a ton of creative ideas for kids and shows you how to adapt them age wise and for travelling. Good luck on your trip!