La Sagrada Familia
To visit Antoni Gaudis most famous work which still isn’t finished yet, you should book your ticket in advance to avoid hours in the queue. Next, be prepared to lose your breath when entering the church, especially in the morning and evening. The pictures try to show what I mean but no picture in the world could render what you can see when being there.
You can also visit one of the towers and get an audio guided tour for free which is quite interesting but if you already had a good view over Barcelona I don’t think it’s really worth to pay around 10€ extra to go up one of the towers since most of what you’ll see is gonna be a building-site until 2026 when they plan to finish the building.