snacks! lots and lots of snacks!! Also, we find it so helpful to have a travel potty (we use a $5 ikea one) so we can just pull over to the side of the road vs getting all the way off the highway to use a rest stop.
If she still naps, maintain the regular nap routine (just in the car.) Around nap time, we put the radio on soothing music, put all toys away, hand out their blankets, and bam - asleep. We planned stops every 3hrs or so to go to the bathroom/get a snack/stretch our legs.
Sticker books are awesome (search for Ultimate Sticker books) - my kids still love them and will spend a good long while doing them.