You should learn how to use apps, as they are completely cashless. If you use cash, you'll receive a bad treatment. Haha

I've been there three times for work related reasons, and each time has been a terrible experience for me.

Here are some warnings to consider when traveling to Beijing:

1. In Beijing, 99% of the PRC does not speak English. You should learn to speak and read Chinese, or bring someone who does.

2. Stay at the tourist place! Do not stay far away from tourist places. To communicate with PRC is difficult, and they usually ignore requests for help. At least you'd get some help from the whites who stay as expats.

3. Do not speak Cantonese! The police have stopped me several times for speaking Cantonese, posing amusing questions such as "Why are we here?" , "Are you a Taiwanese?" Some have even made fun of us.

If you spoke Cantonese, some cashier would throw the receipt to the floor.

4. Be aware of your surroundings, PRC often move around without noticing you. People frequently push your shoulders without apologizing.

5. They have the worst public toilet. No bidet and dirty. You'd notice that the toilet was littered with tissues with poops. Every time I used the toilet, I would feel like vomiting. The smell is terrible. 

My advice to you, PRC, do not like HK-er and Taiwanese. They will treat you as one of them. Make yourself not look like one of them, or else you will receive terrible treatment.