Traveling through Beijing from Thailand on way back to United States

Hi there, I am traveling back from Phuket through Beijing and have an 8 hour layover before heading back to United States. I would like to leave the airport and explore for a couple hours. Can I do this with the 72 hour visa? Do I need to do anything before I go in order to leave the airport or just tell the agent when I am boarding in Phuket and go to 72 hour free transit counter in Beijing when my son and I arrive?
Thanks!