Lagos is one of the more pleasant towns in the western part of Algarve coast and is surrounded by beaches on both sides. The beaches are excellent, although as it's the Atlantic coast, the water is a bit colder than in the Mediterranean - but also cleaner - and still perfectly suitable for swimming, even in the low season.
To the east, beyond the marina, there is Meia Praia, the biggest Lagos beach; a huge sweep of a white, fine sand with a gently sloping sand, far-stretching shallows and a wonderfully airy, open aspect. It can get quite windy, with sand being blown about and into the eyes, but it's a great family beach nevertheless, and as it's so large, it will have enough space even in the more crowded periods.
It's a bit of a walk from town to the beach (it can be reached over the drawbridge and through the marina area), or there is a bus, as well as a direct road with (limited) parking spaces.
Meia Praia is bordered by dunes, which can provide some protection from the wind, and apart from a selection of bars and restaurants is not cultivated at all: if you expect showers, toilets and other facilities, you better look elsewhere.
East end of Meia Praia has water sports facilities, including windsurfing.
On the western side of town, there is a number of coves enclosed by rocky cliffs, each with its own beach, and with its own name. These include Batata (Potato beach), Estudantes (Students' beach), Pinhao, Dona Ana and Camilo beaches. All are accessible from the cliff-top walk and all fairly small, but rather attractive. In low season any of them would be fine, in the high summer season they get very crowded. Still, if you are one of the people who like to try a different beach every time, these coves provide welcome variety.
Praia Dona Ana is the beach whose images turn up on countless guidebook covers and postcards of the Algarve. You reach it by steps from the road level, and it's surrounded by high, golden cliffs on the land side and beautiful rocky outcrops on the sea side. In low tide it's possible to walk to a more secluded cove to the side of the main beach. There are several eating places in and on the surrounding cliffs. The road train running from the marina to Ponta Di Piedade stops at the car park near the steps leading to this beach.
Praia dos Pinheiros (near Praia do Camilo), the last one before Ponta Di Piedade as well as the eastern end of Meia Praia are "tolerated" (i.e. not officially sanctioned, but customarily accepted) naturist beaches.