Located on the north western corner in Spain, Galicia has a long coastline to the north and the the west. Almost 3000km in total.
This large coastline with its “Rias” has a lot of great beaches, most of them deserted making you the only one there. One beach that is famous among pilgrims walking to Finisterre is the Playa de Langosteira.
If you are spending a few days in Santiago de Compostela and would like to have a day by the ocean, you could take the bus to Finisterre and watch the sun set in the atlantic ocean by the Finisterre light house. For bus schedules, have a look at (Finisterre is “Fisterra” in Galician, so use that as your destination).
The Langosteira beach, located next to Finisterre also offers some options for a relaxing afternoon. If you decide to stay the night in Finisterre, have a look here for some nice places to stay.