The day after a marathon, the best sightseeing is flat. Cádiz qualifies: it sits on a narrow strip of land between the Atlantic and the Bay of Cádiz, and the old city is compact enough to walk across in twenty minutes, with most of what matters concentrated near the waterfront.
Trains from Seville Santa Justa run regularly throughout the day, taking around 1 hour 40 minutes to Cádiz station; the station sits at the eastern edge of the old city, a short walk from the waterfront. Buy tickets in advance on Renfe.es, though this is a popular day-trip route and last-minute seats are usually available outside summer.
The Seafront
The Paseo Marítimo runs along the Atlantic side of the old city, from Parque Genovés south to the Castillo de San Sebastián lighthouse at the peninsula's southern tip. It is flat, wide and entirely open to the sea: the kind of route designed for exactly the pace a marathon runner manages on sore legs the following day. Allow one to two hours to walk the full length without pressure to see anything in particular.
Playa de la Caleta, tucked between two 18th-century sea forts on the Atlantic-facing side, is the city's most central beach and was used for the Casino Royale opening sequence (standing in for Madagascar). It faces west rather than south, so it catches afternoon light better than morning.
The Old City
Cádiz Cathedral, with its characteristic ochre-coloured dome, stands directly on the seafront and was begun in 1722 after the city became Spain's primary port for Atlantic trade. The tower (Torre del Poniente) gives a view across the peninsula from a modest height; the flat city geography means this is genuinely panoramic rather than obscured. Entry includes the cathedral floor, crypt and tower.
Mercado Central de Abastos, a 19th-century covered market on Plaza de la Libertad, is the best single stop for the city's food identity: Atlantic fish, local shrimp (camarones de la Bahía) and the dried tuna (mojama) that Cádiz has been producing since Phoenician settlement. Worth visiting before 13:00 when the stalls are fully stocked.
Food
Cádiz is one of the best cities for fresh seafood in Spain. The most specifically local dish is pescaíto frito, fried mixed fish (often including anchovies, cuttlefish, prawns and baby squid) in a light batter that uses chickpea flour alongside wheat flour, producing a texture different from fried fish elsewhere. It is sold from freidurías (dedicated frying shops) as well as restaurants.
Jerez de la Frontera, around an hour from Seville by train (and served by trains that continue to Cádiz, making a two-stop trip possible), is the alternative day-trip choice: the sherry bodegas and the Real Escuela Andaluza del Arte Ecuestre (equestrian school, performances on specific days) make it distinctly different from the Cádiz waterfront. It involves less walking than the Cádiz peninsula circuit.
Getting Back
Trains from Cádiz to Seville Santa Justa run throughout the afternoon and evening. The last train is typically in the early evening; check the Renfe schedule before committing to afternoon plans that run later than expected.