The Tibidabo area is another excursion that yields a nice break from being within the city and is better planned for the weekend when skies tend to be less tainted with smog. (See my Additional Tips journal entry for further details).
Unless you're traveling with young children, the Fun Fair Amusement Park atop the mountain won't be much of a reason for making this trip. Your priority for coming should be the Temple de Sacrat Cor (Temple of the Sacred Heart) that is actually two churches built on top of the other and, at night, are illuminated in white seen from anywhere within Barcelona below. The outer structural works, carvings and statues are more impressive than anything inside and can be better seen from outdoor terraces on several different levels. However, no trip here would be complete without taking the elevator ride (which is almost easy to miss as it's not well marked) for 200 pesetas to the stairs leading to the base of the Christ statue of the cathedral dome.
If you're lucky, the crowds will be sparse giving you an uninterrupted, 360-degree view of the surrounding areas. The mountain of Montserrat is easy to detect off to the Southwest, there's more rolling hills and mountains to the west and north. And to the east, the entire metropolis of Barcelona spills out into a glistening Mediterranean reflecting the afternoon sun. And while endlessly gazing, keep in mind that the name, TIBI DABO, comes from the Latin translation of when the devil took Christ to a high place and tried to tempt Him. "All this I will give you if you will fall down and worship me."
The nearby Torre de Collserola, another abstract constructed transmission tower, is also open with an observation deck, but the Sacrat Cor observation deck appears higher and more centrally located...and definitely cheaper.
Other than the "tempting" views from Tibidabo, getting to/from here is the other bonus of the trip. First, take the FGC line which leaves from the underground station where La Rambla intersects with SE Placa Catalunya. (Most maps make it look like the train leaves from the SW corner!) A one-way ticket to Avinguda de Tibidabo was 145. From here, follow the crowds above ground to where you'll board the last remianing blue-line tram/trolley car. It's a very ambling ride curving up the side of the mountain through mansions, estates and landscapes from various time periods that definitely left me making mental comparisons to several places from around the world...including the Caribbean! The trolley stops at the base of another funicular which will take you to the top of Tibidabo should you chose not to walk on one of the many trails. Return is the same way.