Atins is a small but expanding village of sandy streets amid dune vegetation at the mouth of the Rio Preguiças. It has an ocean beach, a sizable foreign population and a surprising number of pousadas (guesthouses) scattered among its dwellings. It's a good base for visiting some of the more isolated parts of Parque Nacional dos Lençóis Maranhenses – you can have many gorgeous dunes and lagoons to yourself.
On the far bank a 15-minute boat ride upriver from Atins, Caburé is little more than a handful of rustic pousadas and restaurants sitting on an isolated sand peninsula between the lower Rio Preguiças and the Atlantic. It exists solely to serve tourists (mostly those on day trips from Barreirinhas). It's a nice enough place for lunch, but you're much better off sleeping in Atins. If you do choose to stay, you'll have it all to yourself after lunchtime.