What about an AI that asked you questions (especially about your surroundings and the environment ahead of you) and generally engaged you to stay awake?
Even an audio-only version of Duolingo that taught and tested you so you felt productive rather than as though you were wasting time with a prattling AI?
You have to be sure that whatever it is doesn't engage any "spacial" thinking, however. I remember reading about studies that demonstrated that giving people visual descriptions or 3d puzzles was more distracting than other kinds of thinking while driving.
While I sometimes just want to relax, there are times when I'm feeling I COULD be productive if there were something I could do that didn't involve my hands or eyes. :) One thing I've done is to talk into a voice recorder. But there is only so long I can talk about an idea in a useful manner.
Yes, interjecting with questions about the road, traffic in the area, terrain ahead and so on could be helpful if it wasn't too repetitive. I think it would have to be personable and engaging ("Her"-style) to be really effective.
Even an audio-only version of Duolingo that taught and tested you so you felt productive rather than as though you were wasting time with a prattling AI?