In my experience, children either belong to the subset of children who happily nap/fall asleep in the car, or they belong to the subset that don't unless they're very very tired.
As far as techniques for encouraging it, either silence or very quiet non-verbal music can sometimes help, and definitely a window shade can help. If your child likes looking out the window, though, he/she will likely dislike a window shade, so be prepared for an argument there. Otherwise, just making sure your child is comfortable, particularly in regards temperature (not too cold, in particular) will usually be all you can do.
As far as being okay to stare, sure, why not? Lots of interesting things happen outside a car - other cars/trucks/etc. passing by, trees, colors, etc...