When I'm in a car for longer rides (like the one I'm about to embark on) I usually do one of two things:
I can rarely do homework, and sleeping is nearly impossible unless it's dark, the middle of the night, and has been almost 24 hours since my last shut-eye. I do usually listen to music, though.
I call the daydreaming part "car wandering," because my mind wanders all over the place. I keep a notebook with me in case I get some fantastic story ideas, which I do. What about you? Do you read in the car? Daydream? Listen to music?