Funny you should ask about this, as I just finished organizing my Barcelona photos from my last trip there. I took my Contax G2 with 21, 28, 45, and 90 lenses. The lens I got the most use out of was the 21, followed by the 90 for street shooting. Yes, you might want perspective control, but unless you've been to Barcelona before and know the city well, you're not going to be wanting to drag a tripod and a Speed Graphic around with you to take up all your time with correcting perspective and fussing with depth-of-field calculations. Too much great stuff to see and places to go and things to do. The G2 outfit was perfect for the trip, as it allowed me to photograph indoors in low light, handheld, with exclusively shooting Reala. There were maybe a half dozen shots I had to skip out of seven hundred or so frames I shot because my film wasn't fast enough.
In regards to your street question, it depends on where in the city you are. If you are in the Barri Gotic, yes, you'll want to carry your wide lenses. Out in the Eixample, the newer area with a grid layout and wide, Parisian boulevards, not so much, although the sidewalks aren't terribly deep so trying to shoot the facades of various buildings will be a challenge without either going wide or crossing the street and shooting long.
Don't forget that there is a lot more to Barcelona architecture than just Gaudi. Take the time to tour the Palau de la Musica Catalana, the concert hall designed by Lluis Domenech i Montaner. It is one of the most stunningly beautiful concert halls in the world, and the acoustics are up there with La Scala, the Teatro Colon in Buenos Aires and Carnegie Hall. If they are having a concert while you're there, do yourself a favor and attend a performance. It was a truly sublime experience, listening to Sir Neville Marriner conducting the Academy of St. Martin in the Fields in that space. And don't forget to eat at the snack bar before the concert - they have some excellent tapas and great cava to drink, at a far more reasonable price than you'd expect.
For people-surfing, there's lots of good places - the aforementioned Rambla, the St. Josep "La Boqueria" market just off La Rambla, Park Guell, the waterfront by the Columbus monument, and take a trip up to the top of Montjuic to check out the fortifications and the park surrounding them. When I was there, people were biking, hiking and there was even an archer getting in some target practice.
Even though it's a tad touristy, the food is well worth it at Els Quatre Gats, which is in the Barri Gotic, not far from the Palau.
You can't photograph in it, but another very worthwhile visit is the Museu de L'Historia de Barcelona - they were doing some excavations inside the old Roman city wall and found the foundations of a whole neighborhood buried under the Archbishop's Palace. So it's an entire underground museum, below the foundations of other Visigothic and Medieval structures. The exhibit is very well prepared and laid out - do get the audio guide as it is very helpful.