Back to Larrude's original question, no - you don't have to return the car at the end of the free period and re-rent for the paid portion. However, Tino's experience shows that returning it may work out cheaper: take the free days (and a mid-size), then return the car and get a cheaper rate, especially if you go with a smaller car.

I've used the similar approach when renting for, say, 10 days - a week plus a weekend. If I go with a single rate, I'll get the weekly rate plus three single daily rates. By returning it after a week and picking up a "new" car at a cheaper weekend rate. Often I suggest that they give me the same vehicle - they save on cleaning/washing and I don't have to move my luggage!