This does require some pre-thought and planning.
Weeks before our trip, I have 2 small tubs that I pack with craft supplies separated in ziplock bags.
Large plastic gems
foam or wood craft shapes
Pack adhesives that are not messy such as
Also pack markers, pencils, color books etc.
During our trip we have quiet time several times where the craft supplies are taken out and worked on. I have a kitchen timer that I use to time the children. When it goes off, They are rewarded by treats or pulling over to eat at places like McDonalds, etc.
When not working on crafts, we play family travel games such as I spy, and sing songs.
I have traveled with both my chidren by myself for 800 miles, and this made the trip run smoothly.