When we were in Barcelona with the kids last October, our best trick for seeing cultural sights was to wait until they were both napping (D in the sling & E in the stroller. Sometimes it worked, sometimes it didn't.
Here's a picture of newly-awake D crawling through Gaudi's beautiful Batlló House. We're glad she woke up. She was fascinated by the beautiful tile mosaics and we enjoyed showing them to her.References:
Barcelona With Kids
Gaudi's Batlló House with Kids