I would sleep in the hotel for a whole week just to be able to have lunch every day in this restaurant. It is one of those places in which, as soon as the dishes are served, you feel the 'love' with they were made.
You will not taste a better tuna. One day in Zahara de los Atunes has no sense without visiting Antonio's restaurant.
If you come over, here is the plan: check out the forecast, in case there's easterly wind I would not advise to come. Then, call to make a reservation, get to Zahara around 12:30 and go to the beach close to the restaurant. I think it is one of the best beaches of Spain, with kilometers of white and fine sand that has a perfect temperature in summer. The best of this beach is that it's not full: sometimes, you can even be alone 50 meters round. Just a pleasure. Crystalline water, of course.