The Espace Moulyna in the Fida Boulevard is a modern establishment offering milkshakes and paninis. After a few days in Casablanca, eating nothing but couscous and tagines, sometimes I want to eat something a bit more European. The paninis are a bit small, but they only cost 1.50 euros - in the posher neighbourhoods, they'd set you back 4. The smoothies cost 1 euro. They always add sugar, so the smoothies can be very, very sweet. I like the bright colours inside the shop.
If you're out kite-surfing, camel-riding, or just enjoying a walk along the beach of Essaouira, you'll probably need some refreshments! Out at the edge of the beach there are several places to eat, but the best in terms of food and price (but which, unlike some of the others, does not serve alcohol) is the Café Panoramique. It's right on the beach and you can have a drink and snack watching the kite-surfers whip around the waves.
It's fairly cheap (I.e. Moroccan prices, not European prices as happens in tourist-heavy areas) and you can get a drink, chips, Moroccan salad, and perhaps a hot sandwich for 4-5 euros in total. I'd recommend it!