Jemaa el-Fnaa really comes alive at night. It's loud, chaotic, and will have all of your senses working overtime. As you wander through the square, you'll come across groups of locals singing, performing, and playing games that draw crowds of people in. It honestly reminded me of old Las Vegas, where there's always some kind of entertainment happening around every corner.
Pro tip: the warnings about cameras are true. If someone catches you taking their photo, there's a good chance they'll ask you for money, so we left our nice cameras behind and just used our phones when needed. It is definitely one of the more touristy parts of Morocco, so if you're looking for a quieter, more authentic experience, I'd only plan on only spending a night or two in Marrakech.