Oh yes, definitely get them to research things to do, it'd be fun.
The Yelmo Icaria cinema in Port Olympic area shows undubbed films, if you're wanting a lazy day.
The places espanya light show is tacky as anything but cheesey fun, worth a look.
Tibidabo has a funfair and fantastic views of the city.
Of Gaudi, I'd recommend Parc Guell at the top of the city and Casa Batlló near the centre..
The Boqueria market halfway down Las Ramblas is beautiful and fabulous for food, there are little bars inside that serve tapas etc. Otherwise I'd avoid the Rambl altogether.
For day trips I'd recommend:
Girona, one hour by train.
Tarragona, one hour by train.
Sites 30mins by train. Lovely beach, cute old part with church, cool vibes, cool shops.
Port Aventura theme park, one hr+ by train, spend the entire day.
Montserrat, if they're up for a bit of walking. (There is a big monastery, funiculars, zippy train, cable cars).
What kinds of things to your DC like doing?