From the coast of Barcelona… | Photo: Tom Walton.
On foot, there's a lovely walk along the Barcelona seafront. Get off the Metro at Ciutadella (Yellow Line, L4) and the beach is five minutes' walk away. From there, you've got 4km of great beaches to lie on — and a great place for a stroll, a jog, or a leisurely bike ride.
To come back, you could walk, or you can take the Metro from El Maresme | Forum (also L4).
By bike, you can ride way beyond the Forum up the coast, which is right at the end of the Diagonal, through Badalona and Masnou to Premià de Mar (about 16km from the Forum), with barely a car in sight.
In the photo above, you can just make out the Barcelona skyline on the horizon, with Montjuic the hill on the left, the power station at Badalona the chimney just right of centre.
Feeling energetic? You could also take the train to Premià and walk back… Want somewhere a bit less crowded to sunbathe? Including Badalona and Premià you've got six stops with nice beaches (and many more beyond Premià)…