I will have a layover of 12 hours at Guangzhou Baiyun Airport.

Yes, you can make full use of the 12 hours' stopover and tour around the city. It's suggested to go to downtown area. There are several popular attractions, including Canton Tower and Sacred Heart Cathedral. You can also go to Shangxiajiu Pedestrian Street. There are several recommended local snacks. Have fun! Oh, remember to go back to the airport at least 2 hours in advance.