From what I can remember, from memory (influenced by booze):
La Rambla is one of the main tourist spots, but the restaurants and bars which line it are not all that great, and quite expensive, but you don't have to walk far from it to get better/ cheaper etc. Beware of pickpockets around this area mind, or anywhere in Barcelona for that matter, if you stand out as a tourist. each time I've been in a big group (about 5 times), at least one of the group has been pickpocketed, although it's never happened to me. Just keep wallets and phones in front pockets, and secure bags and you should be ok though.
Sagrada Familia is worth a visit, even if you just look at it from outside. Nou Camp is a must for a football fan, a tour is great but a game and a tour even better.
Got ***ed with a mate and My Mrs and wandered off La Ramblas and ended up fighting off (proper toe to toe) muggers who pulled knives.