Beaches near the city
When the city beaches saturate in August, the coast south of town offers wider sand and fewer elbows, connected by commuter train in under an hour.
Castelldefels
A wide, family beach with room even in high season, about half an hour by train from Sants or Passeig de Gràcia. Restaurants concentrate around the beach's back rows.
Gavà and Garraf
Gavà's long wild-ish stretch and Garraf's cove with its seaside barracas are the quiet side of the coast. The train runs along the beach itself, making hop-and-return trivial.
North and coves
North of the city the coast gets rocky: Montgat and beyond toward Maresme for calmer water. For real coves you head up the coast — worth a morning, not a spontaneous dip.