I would agree that Girona, together with the small seaside town of Sitges to the south, are worthwhile day or half-day trips from Barcelona. However, if you're already in Girona, you're only about 30 miles from the small town of Roses, though I'm not sure what public transport is available from Girona to Roses. What's in Roses? Well, arguably just one of the best restaurants in all of Spain. It's called EL BULLI and many people have raved about this restaurant. See
http://www.elbulli.com and for a review visit
http://www.foodtourist.com/FTGuide/Content/I432.htm Having dinner at El Bulli would be a great way to end a day trip to Girona, assuming you can get there by public transport, though you probably have to make a reservation well in advance.