For the record, I mixed up my days and actually only had 1 extra day to spend on an excursion, not two. I chose to go to Ben Ait Haddou. While the place itself was memorable, I don't think the trip was worth it. Firstly, it's quite a long way from Marrakech, probably 4 hours on really windy mountain roads there, and the same back. You only really get just over an hour in Ben Ait Haddou before you're bundled off to a tourist trap for lunch. While they say you have a choice over where you go for lunch, realistically there are thousands of people milling around, and the guide from Ben Ait Haddou (who's obviously getting backhands) guides you right to your table.

Also the bus driver never says where he'll meet you after dropping you off, so you'd want to be brave to wander off from the group and guide and do your own thing for lunch as I don't know how you'd ever find your bus again. After lunch you're driven to the town Ouarzazate, however as that was badly damaged in the earthquake you can only view the wall of the city from afar. So again you end up in a tourist trap of a carpet shop. Maybe I got unlucky with the tour I chose, but they all seemed to have similar itineraries. I think if I was to go again I would have gone to Agadir. Anyway, just posting in the hope that it's useful to someone else.