Gaudi's monuments in Barcelona are easy to visit. On our trip in 2006, we started at the Sagrada Familia. I had been there before (15 years ago), and my impression is that the construction works have taken on some speed. I have memories of an empty church, but now both the exterior and interior have a lot of new features. It even has a roof nowadays. The tourist masses hadn't changed though: this must be the most visited sight in Barcelona (together with Nou Camp, Barcelona FC's football stadium).
Second stop: the formidable Park Güell. Near the entrance, it was also very busy, but further into the public park you can pleasantly sit or walk around. Just to sit on those quirky benches! We also liked the plants (cactuses, yes I know, but we don't see those daily in the Netherlands).
And finally, new to me, Casa Battló. This is one of the fine modernist houses at the Passeig de Gracia, in the heart of Barcelona. A visit to its interior is rather costly (at 16 EUR), but it's definitely worth it. With an audio guide, you can tour about 16 rooms, all up to the top of the building. The house reminded me of the Town Houses of Victor Horta in Brussels, however larger and more over-the-top.