Conclusion: split three days into three themes — Old Town, Gaudí architecture, and the beach/Montjuïc. ① Day 1 Old Town: stroll La Rambla, taste jamón and fresh juice at La Boqueria market, wander the Gothic Quarter and El Born, and visit the Picasso Museum. ② Day 2 Gaudí: the Sagrada Família (book 2–4 weeks ahead for timed entry), Casa Batlló or Casa Milà, Passeig de Gràcia, and Park Güell (book a few days ahead). ③ Day 3 outdoors: Ciutadella Park, Barceloneta beach, the cable car up to Montjuïc for city views, and the Magic Fountain at Plaça d'Espanya. ④ Stay in Eixample, the Gothic Quarter, or El Born; get around on foot plus the metro, and consider the Hola Barcelona 3-day pass. ⑤ Watch for pickpockets, especially on La Rambla.