If you are visiting Barcelona on a budget the tour bus can be expensive and if you don't want to follow the crowds you might be better off buying a T-10 card and explore the underground and bus system. You have 75 minutes to commute between metro, buses, tram, etc. each time you swipe the card. Notice that with this same card you can take the funicular to Montjuic hill from the Parallel underground stop , just halfway up hill (the teleferic, the gondola lift to the Castle at the top is NOT included). For those who want to get to Park Guell, the best stop  is Vallcarca stop (green line) and then walk up to the summit of the park. There are escalators for most of the way uphill to the Park entrance. Its important to note that the tourist bus does not take you all the way to Park Guell, you will have to walk uphill the last 4-5 blocks!

An ALTERNATIVE to the tourist bus if you wish to avoid the crowd but don´t want to take the metro is to rent a bike, scooter or GoCar. The GoCar´s also have a GPS guided city tour so the car will be talking to you, giving you directions and telling you all about the city.

Another option, mostly a trap for clueless tourists who have not much time nor will to explore around, is to take the Barcelona Bus Turistic. It's 19 Euros for one day or 23 Euros for 2 days (you must buy the 2 days at the start). Catch the bus from the Plaça de Catalunya and try to get a seat on the open top deck (if you are able to afford the long queue under the sun). The bus has three routes that are interchangeable. You can hop on and off the bus as many times as you like. There are 6 places where you can change from one colour route to another or just get off at one of the 44 main tourist sites. To complete all three routes will take about 6 hours so day one is pretty full (especially during high season when waits at each stop can be over 1 hour).

The RED Route includes amongst its highlights Gaudi's La Pedrera (The Casa Mila) and his Sagrada Familia. For football lovers FC Barcelona Stadium and for art lovers the Museum d'Art Contemporani de Barcelona (MACBA).

The BLUE Route takes you round some of the cities popular shopping areas, past the 1992 Olympic Stadium and Olympic Village, check out some of Miro's sculptures, the World Trade Centre and finally the Barri Gotic.

The GREEN (Forum) Route concentrates on the area around Poblenou, beach and the forum.

And now on the souvenirs:

First, do not make a fool of yourself: do not buy a Mexican hat.
Those are good for the business of the Hindustani souvenir shops for tourists, but not for the visitor. This is not Mexico, that hat only makes you look like a fool.
If you are afraid of the sun, which you have reason to avoid if you are a pale individual, buy yourself a good hat or cap, and cover those shoulders too: the ones with shrimp red skin the next morning will understand that advice, even if too late. The local nickname for tourists is 'gamba' meaning red shrimp.

Also, don't buy a Spanish flag with the bull and wave it around.
The Hindustani souvenir shops as above will be very happy with you as a customer.
But that piece of cloth only make you look like a clueless foreigner here in Catalonia.

Side note: other than match days, only tourists or children wear football t-shirts of Barca Football Club on the street.

Locals will spot you as a tourist as well from miles away, but at least will not categorize you into the bovine tourist set, and will be more likely to answer your questions and help you.

Have a nice stay and please be very careful of your wallet / handbag. Don't take your passport with you on the streets and don't trust anyone who tries to invade your personal space.