Uh, I misunderstood - so how long will you be on the continent then?
If I only had a long weekend in Spain, not really enough time to add local travel, I would probably spend it in Barcelona. Between the Ramblas, Park Güell, La Sagrada Família,… there are so many sites. For a day trip out of town, we rented a car, went north on the Costa Brava to the Teatre-Museu Dalí, and on the way back found a nice little beach town to enjoy a few hours in the Mediterranean Sea.