I think now would be a good time to book your tickets for the Monumental Zone of Park Guell and Sagrada Familia, since you're just about a month away from your trip, to get the days and times that you want.
In addition to acraven's good advice about transportation, taxis in Barcelona are said to be a good deal - I think even the RS guides mention this. We used cabs a few times and never felt ripped off.
We did get on a HO/HO at one point in our visit, but the cost was so outrageous, we immediately changed our minds. Unfortunately, the bus had already started moving and we had to ride the bus until the next stop - which was near nothing remotely touristic. Luckily, we didn't have to walk very far to flag down a cab.
With T-Mobile, we were able to stay connected in Spain with no additional charges.