This park is very skippable. It's a nice park, don't get me wrong. But its attractions are very spread out and unless one of them really jumps out at you, all of them are skippable. We went to the Miro Museum, the Magic Fountain, the Caixa Forum, the Castle for the views at the top (which are very nice, especially over the port), and took both of the cable cars. The cable cars were fun, but overpriced. The one to the port was the better of the two, but it's also not near anything else in the park, and the signage to find it was not adequate. We did see the Olympic swimming/diving pools on the mountain, but missed the Stadium. Basically, I was underwhelmed by everything at Montjuic, and I wish we had spent that half a day elsewhere.
Also, the signage and directions for transportation around the park was not clear at all. From the bottom near the magic fountain, the best option is to catch one of the buses going up to the Castle, which stops at all the main attractions on the way. There are two cable cars - one from the port to the side of Montjuic, and another from near the Miro Museum up to near the Castle. Both are overpriced, especially the one to the top, when you can take the much cheaper bus that has more convenient stop locations. We never quite figured out the funicular or even saw it. This place is confusing.
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC.