Barreirinhas is the working town at the edge of the Lençóis Maranhenses, and nearly everyone who visits the dunes sleeps here. It sits on the Rio Preguiças about four hours' drive from São Luís, and its riverfront is lined with pousadas, restaurants and the offices of the tour operators who run the park trips.
From town, the classic tour is a four-wheel-drive ride through the river and scrub to the edge of the dunes, then a walk to the Lagoa Bonita or the Lagoa Azul, timed so that you are on top of a dune at sunset. The other standard trip is by speedboat down the Preguiças to its mouth, stopping at the Pequenos Lençóis — a smaller dune field on the river's east bank — the fishing village of Caburé on its sand spit between river and sea, and the lighthouse at Mandacaru, which you can climb for a view over the coast.
Barreirinhas itself is not beautiful, but it is lively in the evening, the fish from the river is good, and the tours leave from the door. If you want quiet, go on to Atins at the river mouth.