What are some of the craft kits you've done in the past? My DD is VERY crafty. She has a fold-down desktop/storage thing that straps on the back of the driver's seat so she can always fold that down and have everything at hand. I've only gone as far as to fill it with markers, stickers and paper.
I have found foam stickers that you can make a "scene" with (like it come s with suns, grass, flowers, etc.) and pack some plain paper to stick them to. There's alwasy ColorWonder stuff in there, plus each trip I add a snack and drink. Pretty basic stuff. I'd love to make things more "crafty" for her and hear what you've done in the past.