Expeditions into the Parque Nacional de Doñana are run by the licensed Visitas Doñana. Your first option is a 2½-hour boat/jeep trip (€35) travelling 30km through the park's dunes, marshlands and pine forests in 21-person 4WDs. There are two morning trips and two afternoon trips March to September, and one daily trip November to February.
The second (less interesting) option is a three-hour hop-on, hop-off ferry tour with a little walking (adult/child €17/9), which runs with the same frequency. Trips depart from Bajo de Guía; book a week ahead.