Layover for a US citizen and will be in Beijing for 6 1/2 hrs

I am a US citizen with valid passport and will be in Beijing for 6 1/2 hrs on my way back to the US from Thailand. Would it be possible to get a taxi to Forbidden City, walk to Tiananmen Square, and taxi back to airport in the allotted time? I don’t care about cost, and would just take some pictures before heading back. Do I need a visa for this?