A big "harmless" thing that is often overlooked by Western travelers when traveling to developing countries is money. From personal experience, a lot of people you meet in China will want to trade currency with you. Keep in mind that China is supposed to have strict currency controls, so this is illegal. Do not swap money at shady venues - you are always supposed to show your passport and you will receive a standardized receipt with stamp and all the details of the exchange.
The currency thing also applies to Russia and countries within its sphere of influence.
Police in PRC is especially kind and outgoing to foreigners. In major cities there is an extremely heavy police presence, pretty much on every block. You can count on a policeman being able to help or guide you to a particular place if you're lost (or at least make an honest effort to). They are unarmed and do not speak English.
Other than that, I can't think of anything else. Do not get involved in drugs as this will end badly.