My girls have always liked the color wonder marker sets - you can get those mini travel ones, they think anything small is cute and fun. Also those magic pen books - they have Dora, my little pony - all sorts now - we bought the Mickey Mouse sets for our trip down.
OH - and I know that many people will disagree but sometimes you get desperate - so I took along the chalk markers and I let them color on the windows of the car... it washed off of our windows with no problems - I saved them up front with me until I needed something - new and different for a distraction.
I did that with a lot of little things, little dolls, a new Polly Pocket, special markers or twistable crayons - and when things got boring - pop out comes something new.
We take a lot of car trips - and the girls have car back packs that I pack ahead of time that they can't look in until we have been on the road 10 minutes - treats they don't normally get, new books to read, color, things to do, just finding all the little fun things I have stashed in to the pack is fun.
Hope I gave you a couple ideas in this rambling post -- have a great trip!!