This tour leaves around 3 or 4pm and drives you out to the Valle, some of them via the Caldera de Sel, which is nice, but mainly for the same view as you get arriving in San Pedro.
They drive you through the national park (they are bound to ask you to pay the entry fee of $2 at this point, even if they promised it was included), showing you various sights, such as the "theatre." They get you to the sand dunes about an hour before sunset to allow you to hike up (not far, but the sand keeps slipping) to the ridge for the view at sunset. This is amazing, and running back down the sand dunes at dusk is awesome. It is hot climbing up, but cold once the sun sets, so have a jacket with you. Then they drive you back to San Pedro.