My favorite part of Spain cuisine is the existence of the "menu of the day". Most places will offer a midday selection of dishes (menu is usually posted outside of the restaurant): you choose a first course (primer plato), and a second course (segundo). Standard menus also include bread (pan), wine (vino) or a soft drink (refresco), and a dessert (postre).
On our second day (I think) in Barcelona, we set out looking for Casa Amalia (Passatge Mercat, 4-6 -- rumored to have a menu of the day for only 7,81€), only to find out that it was closed on Mondays. We went, instead, next door to Pep Restaurante (Passatge Pla, 11) to sample their Catalan cuisine.
The second courses (segundos) we chose were decidedly non-memorable, but the primer plato was to-die-for. Both of us got shellfish soup (sopa de mariscos) -- a tomato-based puree laced with bits of whitefish and tiny prawns. Overall the soup was a perfect balance of savory aromatics and sweet tomatoes and seafood. My only wish was that I had had two bowls. =)
The dessert (postre) we chose was crema catalana (catalan cream), a close relative of France's crème brûlée. Click here for a recipe.