Hi, Us citizen flying through Shanghai

I will be traveling to Cebu, Philippines. My flight flies into Shanghai with a 17 hour layover. My online booking service says I don't need a Chinese entry permit for this layover. However, being that it is so long I would like to explore Shanghai while I am there. I assume that I would need entry permit to leave the airport. Also, my return flight about 2 weeks later has about a 12 hour layover in Shanghai so I would like for my visa to be valid then also. Can you tell me for certain if I need entry permit, which type I will need and generally what I need to do to get it? I live in Commerce, Texas, USA. Thanks.