Our son recommended this place to us. It wasn’t in our guidebook, but it’s very popular with the younger generation especially, who like to go up to the Bunkers to watch the sunset.
We are in our sixties, so felt like the oldest ones there, but if you take the metro L3 to Vallcarca, and the No.22 bus to Gran Turo de la Rovira (penultimate stop), it’s a short 10 minute walk to the top. There’s a set of concrete steps opposite the bus stop, and from there just keep heading uphill. The concrete bunkers were used for gun emplacements in the Civil War. They’re covered in graffiti now, and there are no facilities there, so take drinks with you. The views of the city are superb, and the 22 bus is very frequent, or you could even take a taxi out there, with a five minute walk to the top. We used our Hola Barcelona card on the metro and bus, and the bus stop is very close to Vallcarca Metro station. Worth a trip out for the views. We probably took an hour and a half out of our schedule to fit this in, but were glad we did.