Have you ever played the "punch-buggy" game? You know... the age-old game where you got to hit your sibling if you saw a Volkswagon Beetle/Bug? The game stopped being so novel (but more painful) when VW re-released the "new bug" a while back. Well, here's a different spin-off of the old game (without the punching so no fights ensue).
Pick one color that everyone in the car can recognize. When you see something that color as you drive by, call it out! If you are a competitive family, then you can award a point for each item identified and see who has the most by the end of the trip. If your kids are simply first learning, then use this time to teach them about that color.
Are your kids older? Instead of selecting a color, select a letter (or sound). If your child is working on a particular articulation sound, then that would be a great one to select.
If you are working on shapes, then shapes can be your guide. Find all things that are round, square, triangular, etc.
If your child is working on a particular articulation sound, then look for items that have that sound it!