A few suggestions:
Put your favourite music on and SING!! You can't eat while you are singing. Well, you can, but it will get a bit messy. Singing is really good for you. If you can't sing, well who cares? Do it anyway. I always sing in the car whether I have passengers or not. If they don't want to join in they can walk!!
If you are the passenger, then map read. If you can't read a map, now is a good time to learn. You can pick up a good road atlas from the supermarket or petrol station for a few pounds, and you never know when the skill will come in useful.
Take a plastic box full of slices of apple or other fruit (apple is the least messy to eat if you are doing the driving!), or carrots, or sliced peppers.
Don't shop at motorway service stations or petrol stations. You can make an exception if they have M&S shops - you can always treat yourself to a little pot of melon cubes there!