There's nowhere (and I mean nowhere) on Earth like the Djemaa El Fnaa, the iconic main square in the Medina of Marrakech. You'll realize this as soon as you get off the bus from the airport and stroll along the main drag: the air is filled with the pounding drums of the gnaoua musicians, someone will walk past you with a monkey, you'll small dozens of grills cooking all kinds of meats rubbed with all kinds of spices. It's that precise moment when you realize you're not at home anymore.
During the day, it's fairly calm and is basically an open-air marketplace. But at night, it transforms into a carnival. I'd suggest getting there around dark and grabbing a bit to eat. I wouldn't suggest actually entering the food area; the central part is all tourist-oriented mixed grills and you'll have to battle and endless line of people hassling you to eat at their place. Instead, opt for the cheaper, more authentic, and friendlier roast lamb or soup vendors around the edge.
Next, head over to hear the gnaoua musicians. Don't get right at the edge of the musicians unless you're willing to pay. Stand a few feet off and enjoy the music for free! :o) There really is no where else like it. Thousands of locals haggling, talking, eating, walking, flirting, selling, and just living life. It's an incredible spectacle.
The Djemaa El Fnaa IS Morocco. 100% Recommended!
The Place des Ferblantiers connects the Muslim medina with the Mellah, Marrakech's old Jewish district. The name means "Ironworker's Square" so there are lots of shops around selling decorative iron goods like lamps, mirrors, bed frames, and the like. Some of the bars that line the square also have rooftop cafes so you can enjoy a tea while looking out over the medina. The square was renovated a few years ago and they've added benches, a fountain, and some palm trees. The shops are reasonably priced, but you still need to haggle to get a good deal. Iron working was once the traditional trade of the Moroccan Jews but most of them left the country during World War II.