I also stayed literally on Las Ramblas when I went. Had booked us to stay in a hostel just off of it, but for some reason they had overbooked, so had organized a private apartment for us. It turned out to be huge with loads of room and looked right over the street, but it was a bit of a mission because the owners of the hostel didn't speak any English and took them ages to get the keys from someone who owned the apartment.
I think I went to one of those Irish bars... all owned by the same people I believe. I went in there a couple of times, more just because it seemed like the only place that there were fellow Brits. But we did go right at the end of summer, so it wasn't as busy as I'd imagine it is over the main summer months.
I'd also agree with the comments about being careful where you eat. The first night we were there, we stupidly ate at a place called Genove 1911 on La Ramblas, it was the worst meal in a restaurant I've ever had. My one friend had to leave, he felt so sick. We then had an argument with the manager who threatened to phone the Police. It's funny that I just googled the restaurant and found out we weren't the only ones who had the same treatment!
http://www.tripadvisor.in/ShowUserR...50044370-Genove_1911-Barcelona_Catalonia.html
So avoid that place like the plague!
The ones I heard which were good is the one in the gothic quarter which has a fire oven outside it. There is also one in a big courtyard near the Irish Bar which has a que at dinner time to get a table that stretches across the whole square!
There is lots of interesting architecture to see. The market off Las Ramblas is great, especially for fresh squeezed juices, which are only €1. The beach also doesn't take too long to walk to.
Las Ramblas at night is a pretty interesting experience also. It totally changes from how it is during the day. All the street performers and sellers get replaced by more shady characters. Drug dealers and men selling cans off beer are literally every few steps away. There are also hundreds of african prostitutes who will try and grab you and your mates if you aren't with women. It was like being chased by zombies at one stage when they were banging on the door outside our apartment! It's not right!
Also watch out for pickpockets, as one got my mate and its pretty rife in Barcelona.