Montjuic Park

Montjuic Park (Parc de Montjuic) in Barcelona is a large area for kids to explore - an 18th century castle with commanding views of the Mediterranean, shady gardens with cooling fountains, a museum chock full of fabulous paintings and sculptures by Joan Miro, playgrounds and picnic areas (so bring a picnic lunch and relax outdoors).

Teleferic de Montjuic - Part of the fun of Montjuic Park is the funicular and cable car to go up and down the steep hillsides. To get to Montjuic Castle, you could walk up promenade, or better yet, from the Paral.lel metro stop, transfer to the Funicular de Montjuic going up the hill. Then take the cable car (Teleferic de Montjuic) and ride up to the castle.
From the cable car, there are stellar views of Barcelona (the towers of the Sagrada Familia cathedral really stand out). Also as you ride up, there's a nice children's playground , picnic tables, and other pedestrian paths down below. If the kids need to expend a little energy, after you've visited the castle, run down the hill, stop in the playgrounds, or have a picnic lunch.
Montjuic Castle (Castell de Montjuic) - The perfectly preserved Montjuic Castle (dates back to the 18th century and was in use in the Spanish Civil War as prison) commands an excellent view of the Barcelona harbor and city below. Kids can run over the stone bridge through the gate with heavy studded door, walk around the ramparts, and admire the left-over artillery. The military museum (Museu Militar) is stuffed with sabers, swords, pistols, revolvers, cannons and howitzers, miniature models of castles and uniforms.
Fundacio Joan Miro - If there's one art museum to visit with kids in Barcelona, this is it. The paintings and sculptures of Joan Miro appeal to toddlers and teens alike. Little ones will be delighted with Tapestry of the Foundation , a textile piece in red, yellow, blue and green primary colors with shaggy shapes. The sculpture Monument in the Middle of the Ocean to the Great Glory of the Wind is whimsical and humorous, with a bird sitting in a cut out hole and fork sticking up on top. Teens will empathize with the dripping paint in May 1968 or the black colored Fireworks .
Jardins Laribal - Next door to the Fundacio are shady and cool gardens, with fountains and ceramic tile benches where you can spread out your picnic lunch. (Just head west out of the museum). The Font del Gat spring also has benches for picnics and a charming snack bar.
Poble Espanyol - Poble Espanyol is a village re-creation of architectural landmarks from different provinces of Spain. Step in through an old city gate, stroll through plazas with fountains and streets lined with whitewashed buildings, check out a Romanesque monastery and the cathedral of Santiago de Compostela, there's 117 different buildings. The village also has artisan glass, jewelry, leather and pottery shops. At the educational space for families, kids can paint and do other arts and crafts. On the weekend there are family activities such as puppet shows. Family tickets are available.
Magic Fountain (Font Magica) - In the evening, watch the sound and light show at monumental, splashy Font Magica, a dazzling display of gushing waters and constantly changing colors, accompanied to music. Shows every 30 min., daily in summer, and on weekends only the rest of the year. Access to the fountain is from the Plaça d'Espanya metro stop.