If you can find availability on the website and in your schedule, the most valuable use of free days is on long distance expensive one-ways, with a nice car chosen from an Aisle facility. 7 regular free days can be redeemed together at a max, which may require splitting the trip into multiple one-ways. If so, try to exchange cars along the way at another Aisle facility. Another smart choice is breaking up the trip with a few days of no car if you're visiting a city, such as a cross country drive from Boston to Seattle and several days in Chicago. You can return to downtown Chicago, and then when it's time to resume your trip, take the train and choose your second leg car from O'Hare. To return to downtown Seattle eventually, for instance.