We visited the Arenas before going to the Magic Fountain in July 2013. This was an old Bull Fighting ring which has now been converted into a shopping complex with restaurants. We paid 1 euro to take the glass lift up to the rooftop viewing area where you have excellent views over the city and Placa de Espanya. The views would be even better at night time but it was still light when we visited. I believe it is open until 12am though.
As it was a Sunday, the shops were closed (as are a lot of shops in Barcelona on Sundays!) however, the viewing platform and restaurants are open. A cheap but interesting extra to add onto a trip!More