Quite possibly the most famous tourist stretch in all of Spain, Las Ramblas is a wide, pedestrian mall flanked by large trees along its three-quarters of a mile length. Cutting through the heart of Barcelona from Plaça Catalunya to Port Vell, this is the type of place where out-of-towners mill about in droves and the type of people and businesses (performers, pickpockets, specialty shops, souvenir kiosks, low-quality restaurants, etc.) that prey on them have followed suit. Which isn't to tell you you should not wander this boulevard to the equally touristy and aesthetically revered Plaça Catalunya and indulge that inner tourist; but if you're looking for actual cultural immersion, have a drink at the Plaça Reial just to the east or explore side streets like Carrer de les Sitges at the north end. Even better, divert to the pedestrian walks on both sides of the south end, where you may seek out unique bars like Polaroid Bar (80's nostalgia), Nevermind (skateboarding and rock music), and El Bosc de les Fades (literally translated as "The Fairies' Forest"; you'll see why). Just don't settle for overpriced food or drink on Las Ramblas, especially when you've got great choices a few streets over in the Barri Gotic or Raval neighborhoods.
Note:A cultural centerpoint on Las Ramblas, the Liceo Theater is a renowned opera house placed regularly among the worldly elite. If this is your style, the Liceo is a must.