I was just there for 3 weeks in June! Definitely make sure you put on sunscreen all the time, drink TONS of water, and don't stay out on the beach the whole day. I made the mistake of staying out on the first day from 8-5 and I got so sick and nauseous, I threw up that night. Stay in the shade if possible!
If you're buying souvenirs, make sure you don't pay the first price the owner tells you. Either say you don't want it at that price, and they'll lmost likely lower it, or make an offer to them first. They definitely will rip you off otherwise.
The food from street carts is fine to eat; as far as I noticed, the juices were fine also. I had ice in my drinks and never got sick, but better safe than sorry if you're worried about it.
As far as safety goes, just use common sense like everywhere else. Don't walk around at midnight alone, don't wear a lot of expensive jewelry everywhere, etc.
It's really an amazing country! :) I'm sure you'll have an awesome time!