Do I need a Visa for Shanghai, China

We are US citizens flying into Hong Kong for a 10 Cruise. We have stops in Taipei,Okinawa, Shanghai, and 3 ports in Japan, ending in Japan. From there we plan to spend a week in Japan taveling by local trains, etc and flying out of Tokyo some 3 weeks after starting the cruise.
My question is it appears that Shanghai is the only city where I may need an entry permit (not sure). If our stay in Shanghai is less than 24 hours before we sail to our next port (japan), will I need an entry permit?
If not, what documents will I need to keep in my possession when in the city being a tourist?