The Carretera de les Aigües is the best flat walk in Barcelona, and it's flat because it was never built for walking — it's the service road over a buried water main, cut along the hillside at a single contour. The result is a four-and-a-half-kilometre balcony with the entire city, and the sea behind it, hung below you the whole way.
The trick is the approach. Take the FGC to Peu del Funicular, then the Funicular de Vallvidrera — but get off at the Carretera de les Aigües request stop halfway up, not the top. Press the button; it won't always stop otherwise. You step out already at the height where the view starts.
From there just walk the carretera (road). It's a runners' and cyclists' track from first light, broad and easy underfoot, and the city stays with you at every step — the Sagrada Família, the Diagonal towers, the port, the water. Stop at any mirador (viewpoint) you like; there's no wrong one.
There's no destination, which is the point. Turn whenever you've had your fill, walk back with the view ahead of you, and flag the funicular down.
