While you are in Barcelona, here are a few "must see" places you have to check out:

There is a touristic bus (Bus Turistic) which will take you to most of these interesting places for cheap, about 25 euros for a day pass.

If they have more time they can check out Sitges (beach town)

For restaurants, a nice place is Inopia, a simple but high quality tapas place. Classic places are Set Portes or Botafumeiro.

Here are some additional links that can be useful. The recent NYTimes is a quick read: