Solo, Bunkers del Carmel (Turó de la Rovira) works.
A scruffy hilltop of concrete bunker ruins that delivers a genuinely panoramic, horizon-to-sea sweep of Barcelona — and it's free. Sunset is magic but crowded; locals bring a beer and a blanket. The catch: it's a sweaty uphill walk (or a bus plus climb), gates now close in the evening to curb parties, and there's zero shade or facilities.