If you're willing to hop on a train or rent a scooter and travel between 20-40 minutes outside of the city, Barcelona's coast has some beautiful beaches. The farther away from the city center you go, the cleaner, wider and more inviting the landscape. Many people however don't bother to venture out since there are great options for lying on the sand, surfing and sun tanning five minutes from the Old City. However, if you want to experience a slightly more unique view of Barcelona's coastline, then check out a few beaches, all extremely easy to get to and well worth the trek.
One of the largest and most fun beaches, only 20 minutes north of Barcelona by train, (from Sants Station or Arc de Triumph) is Ocata. Home to rugby and volleyball tournaments this is one of the widest beaches, with fairly white sand and clean water. There's also a string of tasty chiringuitos (beach side huts), which serve tasty tapas as well as larger entrees and pitchers of sangria.
If you don't mind a slightly longer journey, a fantastic hidden gem, is St. Paul's. Unless the school children are having 'beach day' (happens a few times a year) this is a quiet, pristine, beautiful place to spend some time. It's easy to get to by train and a really great way to enjoy a relaxing day under the sun, in a more peaceful setting than the city center. There's a delicious, extremely economical restaurant directly on the beach itself; their 'Menu Del Dia' is around 14 Euros and comes with a bottle of wine and four full courses.
St. Paul's Beach
Another more popular beach, closer to Barcelona, is Castelldefells. Slightly more touristy, yet still very much a local's town, Casteldefells is a nice getaway for the day or even a day and a night as it's always buzzing and full of life. They also have one of the only man made lakes at the Water Sports Club, if you head into the city and is a great place to go water skiing (although very expensive).
If you're bored of Casteldefells and want a more chilled alternative, head to Garraf Beach, which is easily accessible via train. Garraf's a calmer, less touristy spot than it’s rowdier neighbor.