Next was Casa Mila which also had stupid waits, even in end of September, but as good capitalists, they sell skip the line (expensive) online tickets:

one of the building's arches has a view on the unfinished Sagrada Familia

Gaudi used an interesting engineering principle of suspending structures upside down and using gravity to help see ideal building shapes

Down the street was his Casa Batllo, another expensive visit with more expensive skip the line tickets I was able to get:

Another of his buildings was Guel Palace, which we saw just before leaving Barcelona:

another nice roof

And for good measure, the city also had a Gaudi museum: