Most car rental companies offer frequent-flyer miles. These offers are generally for a few hundred miles per day, but occasionally some promotions actually give more value in miles than the cost of the car rental.
One smart person in Las Vegas actually rented up to 10 cars per day, day in and day out, to accumulate hundreds of thousands of frequent-flyer miles at low cost.
During the promotion he would rent a car, drive it to the nearest free parking lot, rent another car, and repeat. After 2 hours (the minimum time to rent at this particular company) he would take the first car to a gas station and add a few cents of gasoline to top up the tank, return the car to the company, then repeat that process until all the cars were returned, etc.
Now this was a very time-consuming process, so few people would be interested in this “occupation.” But if you are going to rent a car anyway, it’s worthwhile checking for the availability of bonus miles so you can fly free.