Do we need a visa to leave the re-enter the airport (one time)and an invitation letter?

My Thai wife (holds an American green card with a Thai passport) and I are flying to Chiang Mai, Thailand on November 18. This trip has us stopping in Guangzhou, China as a overnight layover. 05:40am - Nov 18 to 04:45pm - Nov 19, or about 35 hours. Instead of sitting at the airport, we would like to get a room in Guangzhou for the night and maybe sightsee before we leave for Chiang Mai, Thailand.