I would stop in Zahara de la Atunes, which is approx 30 kms north of Tarifa, and probably about half way. Tarifa is worth visiting in the evening but it is dead in the day as everyone is on the water wind or kite surfing. Zahara is on a very long (20 or 30 kms) atlantic beach, at one end there is an enclave of multi-million euro homes, and in the middle the town (the other end is national park).
We ate at the Hotel Antonio which is south of the town approx 5 kms down the beach. The restaurant there is quite traditional, lots of fish, we had a good meal.
Not certain which way trvlcrzy drives but Marbella is in the opposite direction from Gibraltar.