Barcelona's most famous street runs about 1.2 kilometres from Placa de Catalunya down to the Columbus monument at the waterfront. The wide, tree-lined central promenade is always busy with locals and visitors, and it makes a natural spine for exploring the old city on either side -- the Barri Gotic to the east, El Raval to the west.
The Ramblas changes character as you walk it. The upper stretch near Placa de Catalunya has flower stalls and newspaper kiosks. Midway down, the Mercat de la Boqueria is the star attraction -- one of Europe's great food markets, with stalls piled high with fruit, seafood, and cured meats. Further along you pass the Gran Teatre del Liceu, Barcelona's opera house, and the mosaic by Joan Miro set into the pavement (easy to miss if you're not looking down). Near the bottom, Placa Reial is a brief detour worth taking -- a graceful arcaded square with palm trees, cafe terraces, and lampposts designed by a young Gaudi.
The Ramblas is heavily touristed and pickpockets work the crowds, so keep your wits about you. But it remains a genuinely enjoyable walk, especially in the early morning before the tour groups arrive.