Tian an men square? That's pretty close to the forbidden city and a few malls. The food can be really cheap or really expensive. I had a Chinese girlfriend who often took me out to eat and try amazing Chinese food and I ended up spending maybe 15 kuai (with beers, etc...) However, if you go to the western restaurants (or those for western people) then you can be looking to spend LOTS more. When I first got to China, I would spend 100 kuai in a restaurant.
Street food is really cheap, but it gets boring really fast. I'm Mexican, so I'm immune to street food, but it might be an issue for you.
Also you can't miss the local massages! Find good place (they usually cost about 50 kuai for an hour or more, and you can always always bargain).
If you go to a hair saloon (those with the spinning wheels), be careful because the girls are prostitutes.
I got scammed out of 2,000 kuai when I first got to China and didn't know anyone so I went sightseeing and a girl offered me a massage. She took me to this place where you pick two very cute 18-20 year old chinese girls who will do everything you want for a few hours. What they don't tell you is that they charge 700 kuai per hour plus all sorts of expenses. However, it was extremely fun getting them drunk and having two hot chinese chicks doing pretty much anything I wanted.
China is an amazing place, but you need lots of money and contacts. Knowledge of Chinese doesn't hurt either!