Conclusion: start at Puerta del Sol and walk the historic core end to end. ① Puerta del Sol: see the Kilometre Zero plaque and the bear-and-strawberry-tree statue. ② Breakfast on churros con chocolate at Chocolatería San Ginés, open since 1894. ③ Plaza Mayor, then Mercado de San Miguel for jamón and tapas. ④ Visit the Royal Palace and Almudena Cathedral (the palace is Western Europe's largest — book 1–2 weeks ahead). ⑤ Lunch on a fried calamari sandwich at La Campana, then Gran Vía and El Retiro Park for a rowboat ride. ⑥ Optional: the Prado Museum for Goya and Velázquez. ⑦ Dinner at Sobrino de Botín, the world's oldest restaurant. It's all walkable — no transport needed.