Money can be exchanged in China. Rates should be pretty uniform. I usually exchange in China.

Clothing, when are you going? Assuming soon, it's summer, pack for intense heat and one or two things for rain. You can always buy cheap or poor quality clothing that will be disposable. The Silk Market as well as Yashow (both in Beijing) will serve you well here. Bargain heavily, as in don't pay more than 10-20 percent of the starting price, usually leaning on the 10 percent side.

Electronics in Beijing: Zhongguancun and Nurenjie (mostly 2nd hand/off the back of truck mobile phones, great selection, inspect closely, bargain HARD)...

Bring: Prescription and non prescription meds (esp Pepto Bismol). Power convertors, an open mind and flexible.
schedule.

Dont: overpack/bring too many clothes. You'll see enough do-dads, gifts and other temptations in China. Have fun!

Also FWIW, try to avoid Badaling like the plague. I prefer Mutianyu as it's nicer, far less crowded and not much further away. Simitai (which I hear is the best and I have not been too) is a good bit further away.

Beijing, skip the Cantonese and try to get some Peking duck (see other posts in this section).

That's about all I can think of, have a great trip!