A positive attitude! A new outlook! New expectations! Freedom from debt back home! (I should stop watching inspirational videos).

Honestly, bring coins, stamps, candy, magazines and other unique items that can be given away freely. Know a reader? Bring a hardcover version of their favorite book. Someone who likes to cook? Bring a cookbook. Whatever is not locally available in your area of China that someone (or their kids) would appreciate.
Take home anything normal that has Chinese writing...Coke cans, cheap USBs, children's propaganda books...shirts with bizarre English expressions.

My passport, driver's license, cash, bank cards, social security card and a cell phone with sufficient RMB on the SIM card so I can communicate when I come back.

I can't think of any Chinese novelties I bring back and forth.

Going home, I would take anything that is specific to Chinese culture or that can't be found in my home country. So that goes for pretty much everything in China really, although I find that Chinese teas and cakes tend to be especially popular. That might be a starting point anyway - together with the obvious silk, baijiu, and pictures! 

maybe some local snacks, that goes for traveling in both directions

On my first few trips back home, I took a few odds and ends.  Weird Chinese snacks, novelties, knick-knacks, etc.  Nowadays, I just take my dirty laundry to give it a proper washing and drying.  No idea how the ayi does it...maybe she just washes them in the local fish pond or something.

Coming back to China, I load up on coffee, OTC meds, vitamins, and supplements.

I would say snacks or food. I get the chocolate bar at the airport before I come here. Other than that I used to buy lots of souvenirs, but I don't anymore!