I would suggest this-
drive from Marrakech to Telout as already mentioned. Last I heard was that the road from Telout straight on to Kasbah Ait Ben Haddou was in bad shape after the rains in May, 2006.
Still have time to go visit that old ruins and return to main road towards Ben Haddau- opening hours vary and it may be closed soon due to deteriation. Fantastic place as this 20th century Gloui Kasbah has some rooms inside that are in perfect shape. The wallpaper and plaster of paris etching look good.
*Stay 1 night at Ben Haddou (about 18km before Ouarzazate)
Next day, visit Todra Gorge- nice place for lunch and travel on to Merzouga- the Erg Chebbie sand dunes. These are the largest ones in Morocco.
*Stay at one of many little hotels along the dunes.
This is the best time to visit the desert- so stay one day here and visit the local villages. If it is market day, go to Rissani where many nomads come to shop
*That night, go on an overnight camel trip into the sand dunes and sleep under the stars after a campfire dinner.
Returning to Marrakech requires an overnight stay.(unless you return by bus)
* Ouarzazte or Skoura are possibilities and pass through the high Atlas Mtns in the opposite direction. A couple of nice little ryads here in addition to a lot of 4 and 5 star hotels.
4 nights: Day 5 - back in Marrakech . You do not need 2 nights here again.
Cascades Ouzoud- when I was there, I did not see any accomodations. I am sure others will make a comment .
Most folks arrange driver/guide for trip to the desert and Todra Gorge.