It is incredible isn't it? Supposedly finished in 2026, which is when ill go and see it again.
Id also recommend Casa Batilo, and Parc Guell if you like Gaudi architecture. Even if you don't, they are incredible to look at. The gothic quarter is a good place to get lost in. I suppose you have to visit La Boqueria, about halfway down the tourist trap of La Ramblas. Then there is the seaside, the Olympic stadium on the hill, the cable cars and the football stadium, although as that's being done up there would be no tour. The trophy room is impressive! All of this before you even talk about the food and fantastic weather.
You can't go wrong really. I love the place