You need WikkiStix! We just got back from a 4 adult, 1 6 year old road trip and everyone played with the wikkis! They're super fun for dot to dots, making 3d creations etc.
We don't take a lapdesk because stuff falls off in the car. We use cookie sheets instead. DS can balance it on the arms of the booster and crayons, pencils etc don't roll off onto the floor. Also, magnets stick so he can draw on plain paper magneted on so it doesn't slide, or he can use magnets to play with on the sheet.
We also found generic cheap madlibs at the Dollar Store that comes with a wordbank. It makes it a bit easier for littler kids who can't think of 10 adverbs. I think they're made by Beaver or something like that. We play lots of ABC games (this trip we did fruits and vegetables- someone says the first one "apples" then the next person has to name two "Apples, bananas" and so on).
