I will travel to Japan in April with my wife by Air China. We will have a 12 hour stopover in PEK.

Yes, you can make full use of these hours to tour around downtown area. Upon landing, it's suggested to take airport express to Dongzhimen Station. Then transfer to subway line 2 to Qianmen Station, Exit A. After arriving, walk to Tian'anmen Square. You can also visit the Forbidden City. Wangfujing Commercial Area is nearby. You can find nice local snacks there. Notably, remember to go back to the airport at least 2 hours in advance.