Looking for a fun, and informative day out?
If you're interested in science at any level then a visit to Cosmocaixa in Barcelona is a great place to visit.
It's basically a science museum with many interactive exhibits that are fun for kids and adults alike. And how do I know? I spent a very entertaining few hours there yesterday.
If you have a car it's simple to get to as it's just off the Ronda de Dalt. If not you can get the train to BCN then take the underground.