Barreirinhas is a little village, economically dependent on the tourist trade, as it's the main departure point for travelers heading to the Lençois Maranhenses National Park. As a result it's touristy and you won't find youself alone if you head out to explore its lagoons. But that didn't bother me as I was too busy enjoying the stunning scenery, worthy of its reputation.
The variation between dunes and lagoons was majestic; white sand contrasting with turquoise water, under a glittering, afternoon sun. If you enjoy hikes, find a local agent to organize a trek across the dunes. It's incredible and you won't have any regrets.
Organized day trips from Sao Luis do Maranhao are also available. You'll have an enjoyable tour of the region for a reasonable price. And if you have a bigger budget, you can charter a plane and do the same by air.