trips to barcelona-nou camp and bull fighhting and aquarium- to monserrat -monastery in mountain side- to adventure parks-puerto aventuro, can all be done.
trip to siesta nightclub at malgrat for best-in my opinion- spanish dancing show.
there are plenty of other excursions, but these are probably best/most popular.
if you go to barcelona you may be better making your own way as you will get more time to see what you want to see , but it depends on you/your childs ageand how confident you are about going out and about in foreign lands
if you are interested at all in plants/gardens a trip to blanes would be well advised

Barcelona in the evening to see the fountains, wonderful