Two companies offer double-decker tours of the city. A full day pass costs €14.00. This allows you to get on and off the bus as many times as you like at any of the 38 stops, exploring all Barcelona’s main tourist attractions. Several buses are running on the same route, so you just wait for the next one to turn up. In weekends and public holidays the buses can be crowded, particularly at the most popular tourist destinations.
If this is your first trip to Barcelona, the tour is an excellent way of getting an overview of the city. Make sure you visit the Park Güell, as getting back there takes time. Most of the other popular sites are easily reached by public transport.