Im just back from Marrakech - had an amazing time. At the most you need 2/3 days to see the main sites of Marrakech:-
- jama al fana - the main square - plent yof markets and entertainers - best to visit after Asr salat and early evening. The Kortobia tower is 2 minutes away. Also not very far are the Kasbah mosque and the Saadian tombs. From there you can walk 10 minutes to the Bahi Palace are whatever its called. the Agdala gardens are not that far and within walking distance of the main square. Its a good walk along roads with orange trees bearing fruit liing the path and palm trees too.
You can do them in whatever order but the main square is busy after Aasr.
marjorella gardens is in the new city, gueliz, and worth a visit. taxis accept 200 dirhams from the jama al fena.
menara gardens also woth a visit.
spend sometime shopping and walking in the old streets/markets/souks - whatever price they offer you - you say 25% of it.
try organise a day trip to Oukaimeden - or however you spell it. Its in the Atlas and theres a ski slop there. You get a wonderful view of Berber lifestyle as you drive up. You can get taxis to it for 600 dirhams, = £55
its possible to get to agadir from marrakech and it takes a maximum of 5 hours, on the quckest bus service. its quite a good journey through some beautiful landscapes. I think casa is not that far from Marrakech - if its just 3 hours then i would consider it. You only need 2/3 days for Marrakech.