If you take the Tourist Bus, get off at the Sarrià stop. This neighbourhood, like Gracia, was an independent municipality before being annexed by Barcelona, though its pedestrian streets and traditional shops continue to give it a village "feel". Heading down its main drag Major de Sarrià, you'll come to the famous Bar Tomás (Carrer Major de Sarriá 49), which has the best "patatas bravas" in the city (12:00 to 16:00 and 18:00 to 22:00. Closed Wednesdays. Reservations not necessary).
In case the line is too long, in the street below you can find the bar Ivorra 10. It belongs to the same owners and the menu is exactly the same (also the "bravas"). Go there and you will not have to wait.
Getting there on public transportation is a little tricky: In the Plaça Catalunya station, take the S55 railway line (Plaça Catalunya / Sarrià) and after 7 stops, get off at the Sarrià station (you can use your T-10 ticket).
