It isn’t easy to write something original about Barcelona. The truth is that the city is quite diversified, and that no one can feel like he doesn’t belong there. When I plan a trip, I always first read about the top ten or what is a must see. However, I like to dig a little bit deeper and find out the hidden secrets if in this era of mass media something could be called that.
When: May. Where: El Raval. We came to El Prat airport from Bergamo, with late flight and caught N16 bus at Terminal 2 to Plaça de Catalunya. N17 bus goes from T1, and the price is just 2,15E, or cheaper if you take T10 ticket for 10 rides for 9,95E. It is also valid for City Centre Zone 1 on the metro, FGC , buses, Tram and RENFE. For Airbus and metro to Barcelona Airport you would need other ticket. You can also use one T10 for more people, you just validate it as many times as you need.
I was happy to find out that during my visit, night of the museums was taking place on 21.5.2016, so I got the list of all the places which could be visited. Of course, I was not a freak to run from one place to another, though the idea sounded fun, so I decided to visit just one place. Lets be honest, I was supposed to take metro to several locations, and even to see a magic fountain, but my plan failed :) However, I am sure that I made the best choice - Recinte Modernista de Sant Pau. There was a concert, people dancing, tapas and drinks served (for money, of course). I certainly did make a great choice. My list also had Picasso museum, Maritime museum, FC Barcelona museum (don't ask why), but my lack of organizational skills had me stuck at one place, which I don't regret at all. Anyway, a lot of the museums could be visited from 15h each Sunday for free....and no, I didn't manage to use this opportunity.
On my way to metro, made a few pictures of Sagrada Familia, for which I am sure that is on your list, and it should be. For me, it was more beautiful inside where I spent 5 hours on the previous visit. This time I decided to be more low key tourist and just enjoy the not so touristic places.
 
Looking for nice small squares where I can chill I found a part of the town called Gràcia. It is not far from the center, and if you like to walk, you can walk the nice avenue Passeig de Gràcia, admire Gaudí’s Casa Batllo, and Casa Mila, visit at least one of them, and continue walking until the metro Fontana, from where you could turn right and get lost in the Gracia neighborhood.
It is definitely an inviting lively place, with lot of cafes, restaurants, people walking or sitting in the street. Plaça de la Virreina, Placa del Diamant, Placa de la Vila, Plaça del Sol are all nice squares full of life. In the evening, you can buy your drink and sit in the Placa del Sol, listen to guys playing guitar... You cannot get lost, but even if you do, that makes it more fun, because you will always find something more interesting on the next corner. We ended up by chance in Café Salambó www.cafesalambo.com and I ate the best dish by far - Beef bourguingnon served with parmentier potatoes and porcini mushrooms. We learned the hard way what carpaccio was, since we also ordered ox carpaccio, and it was raw meet sliced and covered with some strong mustard and cheese. No way, never again, though somebody might like it. We were not that modern :) 
I always like to see interesting statues and I loved the once which I found in La Rambla de Catalunya, La girafa coqueta, El brau pensador, Font de la Granota at the junction of Avinguda Diagonal with Carrer Còrsega, El Poder de la Paraula in the Parc de les Cascades 
I realized that I made a huge mistake that I didn't spend each day on the beach. Why not. Whatever you do, you can still finish your day enjoying in the sun. The easiest was to catch a metro to Barceloneta and stroll down the street until you find a small paradise for yourself. It could be windy, but there were a few courageous people taking a dip in the cold sea.
We took a metro and got off in Lesseps, followed the signs to Travessera de Dalt and after a little walking we saw the escalators which lead to the park. Few years ago the entrance to Park Güell was free, and huge part of the park still is. However, it looks funny that they put guards all around the central part, pavilions, terrace but I made a few pictures. In order to see the Gaudí's el drac, multicolored mosaic salamander, you definitely have to open your wallet and pay the entrance.  
 Park Güell was just a stop on our way to Barelona Bunkers, Bunkers del Carmel or MUHBA Turó de la Rovira which were made during the Civil War. I believe it gives the best view on the city. The first time we visited it we stayed so long that we missed our bus, and had to find alternative ways to go back. The easiest way to reach the Turo de Rovira is to exit the Park but not on the main enterance but on Ctra. Del Carmel and take the bus 24 or 92, get off after few stops on Ctra del Carmel-Mühlberg. Go up the street on the right and it is the best to ask someone if you are confused. To be honest, you could see the bunkers from below, but each time we reached it using the other road.  I believe it you!. Definitely take some drinks with you, and snacks, and maybe some blanket to sit on, because if you go there, you will not want to leave.
For Montjuic take bus 150 near Placa Espanya at Av Reina Maria Cristina-Pl de L'Univers next to Torres Venecianas, and the bus takes you close to Castell.

Since I wanted to visit Palau de la Música Catalana the best thing was to buy tickets for Barcelona trio guitar and dance Tribut a Paco de Lucia and enjoy the concert in this beautiful place. The price of the ticket was 35 Eur, and it is worth it.
The highlight of the trip was a visit to Montserrat mountain. If you are a nature lover and like to hike, please go there. It is easy to reach Montserrat by city rail, line R5 direction Manresa which leaves from metro station Espanya. Just follow the signs which take you to the trains which go each hour, and sometimes even more often. The last return train from Montserrat is on 18.15h. You will find ticket vending machines in the station  For me the most convenient was to buy the ticket for 29,50 Eur which included train, rack train, funiculars, audiovisual show, and 2 metro tickets (though I used this ticket 4 times in metro without a problem, maybe because we didn't use it for Santa Cova funicular, just for Santa Joan). Before purchasing the ticket you have to decide whether from the train you will reach the monastery with Cable Car or Cremallera Funicular - Rack Train. You buy the ticket which will include one of those and you cannot change. You get off the train at Aeri stop for the cable car or the Cremella stop for the funicular. It is well marked on the train and you cannot get lost. We went with cable car. You have to take care to come in time for return because there could be a lot of people waiting and you might not catch the cable car in time. On our way back we lost the train and we had to wait for the next one, so there was a small cafe bar next to the train station which was nice to rest a little. 
There are two funiculars: the Funicular de Santa Cova and the Funicular de Sant Joan. We only used the second one, though they say that Santa Cova trail is the most popular. I believe though that the most interesting is Sant Jeroni trail which takes the most time. However, we didn't have a lot of time. When you reach the top with Funicular de Sant Joan, if you go left it is Sant Miquel trail, right next to the Funicular station is Sant Jeroni trail and right up the hill is to Sant Joan, and so we just walked a little bit to Sant Joan and Sant Onofre, and also a little towards Sant Jeroni, maybe for a 15 minutes, and the views are spectacular. I know that also the trail to Sant Miquel is also interesting.