All of the above - Gaudi's architecture is unmissable, the markets off the Ramblas have a lot of atmosphere, and the beach is a great place to do some people spotting - equipped with it's very own basic mini gym, but quite entertaining to watch those who love to be watched! Make sure you go up Mount Tibidabo as it gives a good vantage point of the city, and above all, just enjoy strolling - sometimes when I have travelled in the past I have relied on my guide book too much to lead me to some fab find of a bar (and I have been disappointed, especially when you go out of your way). Of course sometimes they are fab, but there is nothing better than walking around a city, getting lost and making your own gem of a discovery - happy rambling!