Here are three to four recommendations for day trips along the coast:
To the North:
San Pol de Mar, 50 minutes by train to the north, very small town, clean, natural beach, some rock coves walking south parallel to train lines, Link here to schedule (be sure to check weekend schedule)
Portbou, 2 hours by Media Distancia MD train, 3 hours by Cercania train, at the French border, crystal clear water, small town, 2 nice coves north of the main beach in town, you can hike the trail but it will eventually end. Very remote town, nice. Train schedule here departing from Passeig de Gracia here where we went to Girona (under construction so walk here on Carrer de Pau Claris) Be careful to read the Weekend schedule and look for key to Saturday and Sunday.
To the South:
Sitges, 20-30 minutes by train, sandy beaches no coves, designer town, small but more developed town than other options above. leave from Plaza Catalunya usually with a change in Sants Station or option to leave from Plaza Espanya, see here.
Vall de Nuria, 3 hours+, located at the base of the Pirenese Mountains, great mountain climbing experience that Andy White did, I think he stayed over in the town of the Ribes de Freser. You take the train to a Cremallera (mountain tram). Info here. and timetable here (maybe a transfer in Vic).
pics of Portbou