As someone who has loved sagrada familia since I went in, I think the experience of Sainte Chapelle is my second favorite (go first thing when it opens to have it to yourself) and is more underrated than sagrada familia^

Related/unrelated, part of my joy in the sagrada familia is that being a tourist feels essentially the same as being a pilgrim. If you get a chance to visit parc guell, you aren't exactly experiencing it as a park, but as a tour through the different ideas in the park. (Compare this with an unguided stroll through Central Park, where you and all of the other visitors are likely experiencing it as a park (the way it was intended)

^ I think! In my experience it's occasionally overlooked in a short trip to Paris, whereas if you're going to just see one Gaudi, make it the cathedral