When we're on a road trip, getting a bit of exercise once we're at a destination isn't always difficult. You can walk around the city or town, national and state parks and forests have hiking trails, and motels can be sought out that have pools and fitness rooms.
However, when you're actually in the car and on the move, you're sitting. And sitting. And sitting. Certain positions, such as propping your feet up on the dashboard for a good stretch, or out the passenger window, are not safe for the passenger in the front seat. (If the driver has to brake hard, the passenger's legs may be slammed into the window.) Of course, if you're driving, you can't change that position.
Doing a little research, I have found a number of articles about exercises that the passenger CAN do in the car, and a few can be done by the driver (with care, of course).
“The Pre-Travel Workout” - Shape (magazine)
“Road Rules: 10 Exercises Designed for Long Trips” - SparkPeople (Live Fit)
“Road Trip Stretches to Keep You Loose” - Men's Health
Stretching Tips for Road Trips - Women's Day
You can stop at occasional state parks that happen to be right next to the highway (there are a few) and take a hike. Or, if you are on an interstate, there are usually rest areas to stop at. These rest stops have restroom facilities, pet areas, picnic tables, and lots of parking.
Interstate Rest Areas can also be used for stretching, walking, and even a few calisthenics - see these articles for ideas:
“Driver Digest: Quick Rest Stop Exercise” - NFI - though this article is aimed at long distance commercial drivers, many points should be considered by those driving many hours in a vehicle.
“Rest Stop Fitness: Easy Workouts While Traveling” - Johnson Health
For those with other issues about driving and fitness, such as traveling with back, knee and foot pain, or under doctor's care for such issues as heart health, diabetes, lupus, and more, there are these articles:
“13 Ways to Stay Healthy on a Road Trip” - Greatist
“7 Tips to Alleviate Back Pain on Your Road Trips” - Spine Health
Taking Diabetes on a Road Trip - Diabetes Management
Traveling with Chronic Heart Issues - Expedia
How to Travel with Heart Disease - US News