There is something to photograph anywhere. My comments are related to security, based on a personal experience.
My ex, a photojournalist for Getty at the time, was kidnapped there by getting into the wrong taxi cab right in front of her hotel. She spaced out while making a business call, and forgot that the hotel had specifically told her to have the front desk call her a cab, not to get into any random one off the street. We re not talking about a ditz here; she is a very self-reliant, intelligent, and street-wise woman. She just had a brain fart due to distraction, and it cost her. Within an hour and a half, Getty had paid a ransom equal to my yearly gross salary to get her released. Don't go around looking like you have money, with fifteen expensive cameras around your neck, or looking lie you are not paying attention (i.e. talking on a cell phone). Wear plain clothes and don't carry much cash. Go out in groups if you can. Avoid shoulder bags and wallets (money clips held in a front pants pocket are great), and be mindful of your backpacks even when they are right on your back. And have your hotel call your cab for you.
It is also beautiful and intriguing, and the people are terrifically friendly in general, and just because it can be dangerous, does not mean you should think of everyone as a criminal. Don't let fear ruin it for you. Just use more "uncommon sense" than you might normally use.
Oh, and I would suggest learning some Spanish. It is an easy language in the grand scheme of things. It is fairly general in nature; there is not a ton of vocabulary. You can say a lot using very little, in other words. Verbs and tenses are always a bitch, in pretty much any language. But you can get yourself through a conversation without actually knowing that much. You won't be able to learn perfect conjugation without being immersed in the language for some time, but at least learn some common infinitives. At the very least, take some time to learn proper pronunciation. There is pretty much only one way to pronounce each letter, so incorrect pronunciation is pretty easy to avoid, and something that simple will help you more than you might think.