People watch and paddle around the lake in Ciutadella Park
Soak up the sun and stroll the grounds of this centrally-located Barcelona park. Home to Barcelona's zoo, the Catalan Parliament, this park hosts interesting events (and political demonstrations) throughout the year. During La Mercé (Barcelona's city-wide festival), there are open-air concerts and events here.
But by far the biggest attractions are Josep Fontseré's massive fountain with its gorgeous central waterfall and sculptures, and a large lake, where you can rent a paddleboat for a few euros.
There's also a silly sculpture of a mammoth that kids (and the young at heart) like to climb.
If you find yourself in need of some caffeine while you're there, in the glass and iron greenhouse (el invernadero) there's a coffeehouse.
The nearest metro stops are Arc de Triomf (L1) and Ciutadella/Vila Olimpica (L4)