I've just come back from there, with my toddler (I am pregnant, didn't fret about food poisoning, just avoided the Djemma al Fna foodstalls and the fishstalls in Essaouiria) I've been about 3 or four times and my main advice is to pack some lightweight, but warm items (cashmere cardis for example) for night. The temperature even in April can fall dramatically at night.
I went in March and actually had quite a tough time in several of the riads (marrakech and on the coast) as the sleeping rooms came off the courtyard and it was barely degrees at night.
You don't have to worry about offending but take something for shoulders in case you travel about and want to stop.
Eat the chicken and vegetable tagine at Chegrouni restaurant (go three flights up) on Djemaa al fna. Although a tourist spot my moroccan friend says it's the best restaurant tangine in marrakech.
Shopping is a complete pain in the arse and I find the quality crap and the prices (even halved or less) silly. But if you fall in love with something (I love the ponyskin animal print babouches) be prepared to go back over several days to get a fair price.
If you want antiques or carpets then be prepared to get royally taken up the jacksie. Take an expert with you.
There is a state-owned tourist shop up Mohammed 5 about 5 mins from the square which you can browse to get an idea of what prices should be.
Oh, argan oil a good buy, still not cheap (can get on eBay, let's face it) but there are coops dotted around. It's lovely for hair.