Am I correct that I do not need any additional Visa as long as I stay 72 hours or less?

I work for a major airline and have a U.S. Passport and will be visiting the Shandong Provence -Shanghai. If I were to need to stay one additional day where would I get onelocally? and if I fly to Japan after 72 hours and then come back to China 24 hours later may I then stay another 72 hours?