Doñana National Park — one of Europe's largest and most important wetland reserves — sits directly across the Guadalquivir from Sanlúcar, and the river crossing is one of the most satisfying ways to experience it. Boat trips depart from the port at Bonanza, about 3km north of the town centre, and take you up into the marshes along the park's edge. You're likely to see flamingos, spoonbills, herons, and, if you're patient and quiet, Spanish imperial eagles.
The guided trips from the Real Fernando boat are the standard option — four-hour excursions into the protected zone, with bilingual commentary. The park itself has no roads accessible by the public, so the boat is genuinely the best way to get close. Book in advance in spring (migration season) and summer; the trips are popular and capacity is limited. The port at Bonanza is worth a visit in itself — a scruffy, working fishing harbour with good cheap seafood at the dockside bars.
story: "The Guadalquivir estuary at Sanlúcar was the last piece of Spain that Ferdinand Magellan saw when he set sail in 1519. He never came back. His ship completed the first circumnavigation of the globe three years later, but Magellan himself was killed in the Philippines in 1521."