If you can, pay the extra to go straight into Marrakech as going in via Casa is more hassle and time that detracts from the fun of your holiday. This is especially if you have to fly via a European hub first as it is all airport, airport, airport,
Marrakech is "THE" experience of Morocco and honestly very "full on" - just please hook up with local guides, probably and preferably via the hotel, who will deflect some of the more 'enthusiastic' locals who might make life a bit tedious. There is a huge amount to see as it will be so totally alien to you. We loved it and went back time and time again but the guide will be a very useful "shield".
Essaouira, like PortM suggests should be very pleasant as it will offer a coastal stay. I suspect that it has expanded greatly since we were there so I am not really up to date on that.
Personally, I would probably opt for Meknes as the 2nd centre as it has a lot of history and "son et luminaire" shows. You final choice will probably depend on the amount of "history" that you like to have. Having said that, I recall that Marakkech also has a very pleasant Club Med hotel in the centre of town that could give you a seaside feeling in the middle of town. Who knows?
Keep asking, we will do our best .