Do I need a transit visa to leave Beijing airport during 8 hours layover?

Yes, you need a transit visa as you leave the airport to visit Great Wall. It is costs CA$50 for Canadians. If you don't have time to get a single entry transit visa to China, you can try to get a 24 hours stay permit when you arrive in Beijing airport. Please show your connecting flight tickets to the immigration officer to prove the real transit. However, I can't sure you can get it.

I actually think this answer is no - if you are a US Citizen and in the country for less than 48 hours, it doesn't seem you do. https://www.travelchinaguide.com/embassy/visa/transit.htm

US citizens are not allowed to get a visa on arrival in China, in fact, 24 stay permit is a kind of a transit visa for aliens who transit China for less than 24 hours, therefore, US citizens have to obtain a transit visa in advance if they want to exit the Beijing airport or stay for more than 24 hours in the airport. If you are a Canadian and the layover in Beijing airport is less than 24 hours, it is possible for you to get 24 hours stay permit.