It's been faaaaaaar too long since I've done a Weekend Wanderlust post. When we went to Barcelona last year we took a day trip to Montserrat, which is in the mountains near(ish) the city. FYI, Wikipedia tells me that there's also a Caribbean island called Montserrat, but that would have been a little impractical as a day trip from Barcelona, y'know?
As we'd both been to Barcelona before, we decided that we'd spend one day of our holiday outside of the city, so that we could do something totally new. It wasn't really beach weather, so we plumped for mountains instead. Getting there was pretty easy; you get a train from central Barcelona to the foot of the mountain, then either the cable car or the rack railway up to Montserrat itself. If you're interested, there's some actual travel information here.
Montserrat is famous for its monastery, which is built high up the mountain and, judging by the size of the queue, contains a statue type thing that is loaded with religious significance. It was a lovely building, and definitely worth looking around, but I was all about the views (...bout the views, no churches).
There are a few different funiculars you take to travel further up into the mountains. After I'd explained to Pete for the 50th time what a funicular was, we had a walk and enjoyed the views (once the fog had cleared!).
I'd definitely recommend a day trip to Montserrat. It's something you'd want to do on a dry and quite clear day, so that you can make the most of the views and walks. Tip: once you get there the food options are a bit limited, so take a picnic. Thrifty, innit?