And never ever put your valuables in your back pocket. Your wallet should be in your front pocket when you get off the plane. Just a habit of mine when I travel overseas. I think only in Japan I'm okay to put at my back pocket.
And never ever follow anybody anywhere in China if you do not know the person. If someone comes up to you and get friendly and chitchatty, please be cautious of your belongings and don't follow them. Whether it's a young girl, boy, guy, woman, uncle, grandpa.
Never go China when they are having holidays, unless you like to experience the crowd. After being in China cities so many times and squeezing in their public subways. I find our Singapore MRT rush hour crowd is nothing hahha...
The subway is very convenient in Shanghai. In Hangzhou you'll probably need to take the taxi around unless your hotel is near West Lake (Xihu).
When taking a taxi from a high speed rail station or airport, please use the official taxi stand and ignore the touts.
Foreign credit cards are usually accepted at major hotels and restaurants. Cash would still be the most convenient so always bring cash with you.
Where will you be staying in Shanghai and Hangzhou?
Oh yeah, and do enjoy your trip even after we've mentioned so many comments. China is a great place to see the historical stuff.
Hangzhou can see West Lake and Yue Fei's memorial (north side of West Lake).
I find that when I travel I walk alot and use public transportation the most (subway). Unfortunately I can't take the bus cause I can't read chinese characters. T_T