You can easily visit the Sossusvlei (and the nearby Deadvlei) in one day. The park opens at sunrise. Many people get to the Deadvlei really early to take pictures. But the sunrise at the Sossusvlei is also a real natural spectacle. The light falling on the dunes is phenomenal. And the dunes are perfect for a picnic or just to enjoy the surroundings.
Do keep in mind that you’ll already encounter many high dunes in lots of bright colours on the way to the Sossusvlei. You’ll also see lots of animals such as ostriches and different kinds of antelopes. So the road there, and back, will also be worth taking at your leisure. Do make sure you’re out of the park at sunset, otherwise you might get fined.
Tip: See the Sossusvlei in bloom
In springtime, you could get lucky. After the heavy rains, the lilies in the Namib desert start to bloom. That includes the Sossusvlei, and this creates incredible contrasts. The ground will be covered in white flowers, against a backdrop of dead trees, red sand and (often) blue skies. This flower explosion only last for about a week, and it very hard to plan for. The most likely month to see it is in March.