I think I just found one of the best museum membership passes around: the Carnet Familiar at Barcelona's Museu Picasso. For a mere 15 Euros you and your family can enjoy a yearly membership to this museum that provides insight into Picasso's formative years and his relationship with Barcelona. A Carnet Individual is also offered for a 10 Euros, also good for one year. All we needed was our DNI/NIEs (Spanish identification documents) and/or passports.
The kids absolutely loved getting their pictures taken for their "very own card." I love the fact that we can drop by the museum whenever we are hanging out in the El Born. There is no pressure to see the entire museum in one visit. And I shouldn't forget to mention that we can skip right to the front of the line (which can be quite long in the summer).