Lucky you, it's wonderful. I spent two weeks in Morocco in winter 2002, starting and ending in Marrakech. It is about the most properly 'exotic' place you can get to in a short time from here.
Walking about the souk and other main areas yourselves is great. I'd also recommend hiring a guide for a walking tour. That will get you orientated, show you places you might not get to otherwise and, in the some of the most touristy areas, reduce the amount of hassle you get afterwards, as people who want to offer guiding services may have spotted you with the official guide, so back off. I experienced this in Fes, which is a bit smaller but the difference the next day, so degree that people were watching all the new tourists, was amazing. Perhaps less so in a busier season and bigger place.
You have to expect quite a high level of hassle - everyone wants to be your guide and sell you things, including children, who I found the most persistent, as they don't read reactions as well, so don't back off if you look unhappy. I found that striding purposefully, even if I wasn't sure where I was going, adopting a confident and cheerful attitude and replying firmly 'ca va merci' to all enquiries, helped. I also found that giving up, allowing myself to get lost and someone to guide me out again, worked fine (old Fes is a real labyrinth but it's quite possible to get disorientated in Marrakech too). You can always get taxis or calliches (horse-drawn carriages) too.
If you spot something you want to buy when with a guide (they will take you to their brothers' carpet shop etc), hold back and return another time for a better rate without his cut. Haggling can be fun.
I spent time there alone and it is the classic case of 'everyone will approach a lone woman but, when with your DH, they will speak only to him', so you may feel invisible. People won't touch you though, so in a way I felt very safe. I realised that chatting to everyone is a big part of the culture, not just aimed at tourists, so, if you're not friendly, even when feeling a bit hassled, people will read that as very rude.
In many situations the friendliness and curiosity is lovely, very genuine and feels a bit naive. I had some lovely conversations on trains. One young bloke practised his English on me a bit (I struggled through most conversations in my limited French, wish it had been better) then said if I was going to Casablanca I was welcome to stay with him - and I do think it was genuine, not creepy, though perhaps fortunately wasn't going there. You have an advantage as a woman that women will talk to you, they won't to men.
On cultural etiquette, do cover up a bit, people may not say so but will be offended by skimpy clothes. It will be really hot. I'd go out in the morning and evening and retreat somewhere cool at mid-day. Food wasn't as spicy as I expected, the tagines I had were surprisingly bland but the pastries and juices - try fresh pomegranate and, if you can, almond - were fab.