This is a great service to go on either side of the city. Just pay the 32 euros for two days and hop on the bus to see the city and explore. A word of caution, two days isn't enough to see Barcelona really well. There's so much to see, if you go for two days you will leave with a sensation you needed another week or month. A very charming city with lots of beautiful gardens and plazas. Barcelona is super clean if compared to other main points of interest in Europe.