Hike the Alkali Flat Loop

From the US 70 pull into the White Sands Visitor Center. Then continue along Dunes Drive. Eventually, Dunes Drive turns into Loop Drive. At the end of this road, you will find the Alkali Flat trailhead. 

One of the most beautiful and potentially disorienting 5-mile round-trip treks through the desert you will experience, the Alkali Flat Trail leaves you following red trail markers with a black diamond through sandy flats and rolling, blindingly white dunes. You're also surrounded by a missile range, so watch out for unexploded exploded ordinances or other debris that may surface.

The National Park Services states the difficulty of the trail is strenuous. I think it depends on your fitness level and the time of day you begin your adventure. Since you're walking in deep sand, up and down hills, expect your hiking speed to slow down considerably. And since you're completely exposed to the sun you're at a greater risk of dehydration, which is trouble in an area where the shifting winds can knock down your trail markers. As such, I would label the trail moderate to strenuous because of the length in the sand, the sun exposure, and the number of hills you have to climb. Expect the hike to take you about 3 hours, 2 if your intent is to just pound through without regularly stopping to photograph the area.

There isn't any water on the trail.  Bring plenty with you, as well as snacks. It's easy to get disoriented, and heat illness isn't a joke. Trail markers regularly get buried or knocked down.  Always look ahead for the next marker before you move on.