AT FIRST GLANCE, the landscape around Highway 17 through the San Luis Valley might appear incapable of hiding anything. The earth spreads wide, splayed out below 14,000-foot peaks and an impossibly open sky. But off toward the Great Sand Dunes an oasis hides in a stand of tall, skinny trees. Finding this gem on accident feels unlikely; from the highway it’s hard to notice the trees or surrounding buildings unless you’re already in the know. Even after turning into the gravel parking lot it’s unclear what exactly lies inside. But if the sign on the gate reads Sand Dunes Recreation,…