When planning your visit to Barcelona, it is very worth to take a look at the web of the Public Transportation Authority, TMB, to check if it is worth to pay a touristic bus or it is better to use public transportation.
entry, Barcelona has a good and quite reliable network of buses and underground. Fares vary. You can buy a single ticket, which is the most expensive option, or buy day-tickets or a T10 card, which are far more cheap.
In any case, if you decide for public transportation, there are small buses with very interesting. One of them is number 120. With a route that covers Raval to barri de Sant Pere, crossing Ramblas and Born, it will offer you a nice trip around the old town and a good idea of the area neighbourhoods. A plus: no tourists use the line, so you are bound to travel with locals.