Barcelona is a big city, but a great way to explore it is on foot!
Take a day to stroll around the city to admire its architecture, feel the charm of the Sant Andreu district, shop and have enjoy a meal at one of their various great restaurants.
Impressive parks worth visiting are the Parc de la Ciutadella http://www.barcelonaturisme.com/wv3/en/page/380/parc-de-la-ciutadella.html?
and Parc de Joan Miro http://www.barcelonaturisme.com/wv3/en/page/394/parc-de-joan-miro.html?
The Montjuic Castle http://www.barcelonaturisme.com/wv3/en/page/1142/.html is also worth a visit!