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

• Las Ramblas, this you have to see. It is a long boulevard that starts in Placa Catalunya and goes all the way down to the piers. Very colorful, with mimes, street performers, musicians, artists, street vendors, etc.
• La Sagrada Familia, this you have to see. This is Gaudi's masterpiece
• The Gothic Quarter (La Ciutat Vella) area in general, old narrow streets, the cathedral, La Placa Reial, etc.
• El Palau de la Musica Catalana, declared a UNESCO World Heritage Site in 1997
• La Boqueria market, off Las Ramblas, amazing fresh fruit and seafood market
• Other Gaudi work, you should check out La Pedrera or Casa Mila in Passeig de Gracia (one of the main avenues) if you can, and the Parc Guell of course.
• If you have time for more museums, the Dali, Fundacio Miro and Picasso museums are worthwhile
• Montjuic and Tibidabo, both hills overlooking the city with really nice views, recommended

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:

• 36 Hours in Barcelona
• Barcelona Travel Wiki
• Barcelona Travel Guide
• Lonely Planet: Barcelona