The car of the future would have pedals in the leg space for every seat. The passengers may rest their feet on these pedals, but if they need to move after sitting for a long time, or just want to exercise, they can pedal as if it were a bicycle. (The driver would have a set of “bicycle” pedals above the gas and brake pedals, and would be able to put the car into autopilot mode while cycling.) This would not only provide them with exercise- the kinetic energy transferred to the pedals would be converted to electric energy to power the car!
The car, of course, could not rely on human power alone. There would be a meter measuring the cumulative kinetic energy generated by all the passengers versus the amount of energy needed to power the car, and that meter would regulate how much gasoline (or other primary fuel source) is being used, depending on how much needs to be used with the supplement of the cycling.
Such a contraption would conserve fuel while promoting exercise, and would be made even more efficient with self-driving cars, because the driver could focus on cycling instead of driving. The kinetic energy produced by humans alone does not seem like that much, and is certainly not enough to provide a car with the power needed to function, but that little bit of power- and that little bit of exercise, too- adds up to create a cleaner, healthier future.