If you are planning on taking a one day trip from Barcelona, I recommend you visit the nearby city of Girona.
You can go by train or car, and either way it will take you about one hour to get to Girona from Barcelona. During my visits to Girona I have always loved to walk among the historical city center, which is the first place you will come by if you park your car right after entering the city. One of the things I like about walking around are the famous old walls. You can walk over them to get a great view of the whole city.
The Cathedral of Girona is, of course, another of the classic sights in the city that I recommend you don’t miss. It’s very close to the walls, right there in the old city. If you’ve been walking all around the old walls, and you are tired, you can take a few minutes to rest having a coffee and/or a sandwich at one of the bars located in the square below the cathedral. I recommend you eat a serrano (Iberian ham) sandwich with “pá amb tomáquet” (bread with tomato, typical from Catalunya), I always do.
Walking around the old neighborhood you will soon end up in the “call”, the old Jewish neighborhood. This is a beautiful historical area of Girona, and I also recommend you not to miss it. Around this area there are several museums, but to be honest with you, I think you’ll get the best Girona can offer outdoors. It’s just a great city to walk around.
Finally, we usually leave Girona walking next to the river Onyar. You might want to take a different route, especially if you come by train, but I still recommend you to cross the bridge over this river. It’s a great place to take some nice photos.