You can enter it and there is an elevator which takes you up 60 meters above the ground, where you can admire a 360° breathless views. If you look North, you can see the Gothic Quarter and the historic buildings that make Barcelona known around the world: the bell tower of the Cathedral, La Mercè and Santa Maria del Mar.
Of course you will see the Rambla and you will be able to take some amazing pictures, as well as Plaça de la Catalunya, which leds you toward the Eixample District, where you can find the famous church of the Sagrada Familia (the Holy Family). If you look South, you can find the Montjuic Hill, having the castle on the tip, museums, gardens, theaters and the Olympic Ring. And if you look carefully, you will also see the maritime museum (Museu Maritim), which is located at the foot of the hill.
Looking East, there is the sea and if you follow the coastline, you will see the Forum district, having its photovoltaic cell and the tall buildings, the so-called twin towers, near the Olympic Marina. Looking West, finally, the Collserola Natural Park will appear in front of you, having its green lung which is surrounded by the city, having the Collserola Tower and the Tibidabo Amuseument Park being the 2 main attractions of the park.
Four different spots in order to give you a complete image of what Barcelona is like, and let you spend a couple of hours looking around all the beauties.