When we travelled with our (8 month old) baby she also slept most of the time. I think you are recommended to stop and let her out of the car every two hours or so, to stretch her legs. If she was really not happy I would go in the back and either read books with her, or I even breastfed her while she and I were both strapped in (it takes some acrobatics haha, but it is not impossible). As a last resort we brought the laptop with her favorite DVD, which we only needed the last half hour of the drive when she was really fed up.