there is a sandwich shop chain in barcelona called 'pans & co' that is quite good. they use high quality meats and cheeses on baguettes that are just plain great.
it's fast food, but it's a pretty good option for a quick lunch. i used to get the jamon serrano and brie fairly often.
also, the boqueria market is ledgendary and is, in my opinion, the best food market in all of europe. go there, buy cured meats, boquerones (marinated anchovies), bread, cheese, olives, and frutas secas and go have a picnic!
(it's off the ramblas about halfway down)