Everywhere in China (except HK and Macau) requires a visa for US Passport holders.

You can get visas in HK very easily - the more you pay, the quicker you get it, down to 4 hours if you wish. China Travel Service is one possibility for processing the application ( http://www.chinatravel1.com/english/...hinesevisa.htm ) but a Google search shows others, or you could go to the Chinese consulate and handle it all your self (which I think would be not much over half the price that CTS charges, but do you want the hassle?)

In principle, if you wish to visit Shenzhen only (the area immediately adjacent to HK) then you can get a limited visa at the border (not true for all passport holders, but OK for US I think).