The drive out to the sand dunes of Erg Chebbi is pretty memorable in the late afternoon sun.  We stayed at a great desert hotel called Kasbah Mohayut in the oasis village of Hassi Labied (sounds like a camera I know!!) just north of Merzouga.  The evening walks out into the Sahara were amazing, watching the changes in colour of the dunes and the patterns of shadows as the sun got lower.  Just after our first sunset we were treated to a moon rise which was great although not so good for stargazing a few hours later as I've never seen the moon so bright!  A couple of evenings later the moon came out a few hours after dark so we were treated to the amazing view of the night sky with no light pollution!