Hi Virginia
I am not sure I'll be back up and running in April. Keep an eye on the Transport Authority website... Still, the itinerary will be covered in the meantime by a regular bus.
Avinguda del Tibidabo is a nice area in the north of the city, where life is quieter than in the centre. It has plenty of beautiful houses, schools -in fact, I went to one located there in my secondary :)-, restaurants and even a great museum, the Science Museum, CosmoCaixa.
At the end of the avenue, by the funicular station, in Plaça del Doctor Andreu, you have a couple of great restaurants: La Venta and Mirabé, with great views over the city and also a club, for a drink afterwards, Mirablau.
Also, in the opposite side, not far from the beginning of this avenue, at 500 yards from JF Kennedy Square, you find a hidden architectural gem: Torre Bellesguard, a medieval royal residence from Catalan nobility that was later renovated by Gaudi. It's open for visits.
The Esglèsia del Sagrat Cor (Catalan for Church of the Sacred Heart) in itself is spectacular from the outside, but surprisingly small inside. This church, however, it's not popular at all among most of us Barcelonians due to its recent past history. The original site was a humble chapel located in the middle of the mountain, very popular among citizens of the city for decades, which visited on weekends as a sort of "Sunday out" thing, a sort of short distance pilgrimage if you want. Whole families spending the day in nature: picnics, mass at noon, children playing around... here a picture from 1887 during a celebration.
However, this changed later in the early 1940s, after the Spanish fascists won the war (1936-1939), instituted their military dictatorship and forced their whims all over Catalonia. They ordered the construction of the current church to "remind" us Catalans of their power. In fact, the inside is full of allegories to the Spanish conquistadores spirit imposing their will everywhere they went. As you can imagine, for us Catalans this is not a welcome scene so, as in some other sites in the city, residents just turned their back to it. These days it's seldom used by Barcelonians albeit, from time to time, weddings are celebrated there.
Still, if you happen to go to Tibidabo, a visit to the church can be worthwhile, if anything for the views. Talking about views, not far from Tibidabo you have another sort-of-attraction, the Torre Collserola. Designed by renowned British architect Norman Foster which is an impressive telecommunication tower with 360º views over the city and the Collserola Natural Park. It has an observation deck on the 11th floor. Check details here.
Enjoy!
Enric