10 experiences
Barcelona is two cities stacked on one another: the Gothic Quarter's alleys, where nothing is more than ten minutes away, and the Eixample's grid, where everything is a twenty-minute walk further than it looks on the map. Plan around that and the day works.
The Gaudí sites are the constraint. Sagrada Família, Park Güell and Casa Batlló all run on timed entry and all sell their good slots early; the rest of the city — the market tours, the tapas walks, the boat trips out of Port Vell — is far more forgiving.
Two to three weeks in high season, and further if the tower access matters — those slots are limited and go first. Park Güell's monumental zone works the same way. Off season a few days is usually enough.
The old city is; the Eixample deceives. Its blocks are long and its diagonals are longer, so what reads as four streets on a map is often twenty-five minutes. The metro is quick, cheap and the honest answer for anything above Plaça Catalunya.
Market and tapas tours through La Boqueria and Santa Caterina, sailing and catamaran trips from Port Vell, day trips to Montserrat, and the Picasso and MACBA museums. Several of those are the parts of a Barcelona trip people remember longest.