Absolutely do not miss Sossusvlei. I could have spent 4 or 5 days exploring the dunes. From a photographer's perspective, they are a very interesting subject.

You should also try to go all the way into Dead Vlei as well. It's a barren watershed where the ground is cracked and dryed up. As Johan says, it only gets water during heavy rains (very rare). The trees are dying or dead as well and the stark contrasts to the red-orange sand of the dunes is really amazing.

During our visit there we had great sky shows as well. The sunsets were great agains the nearby mountains, but the next morning the full moon (and bright orange) set against the dunes and the early morning blue/pink sky before the sun broke the horizon to the east. I'll never forget THAT image, nor will my camera. ^