If you have 2 days in Beijing,  I would like to suggest you visit:

Day 1: Tiananmen Square, Forbidden City, Jingshan Park, Summer palace.

Day 2:  The Temple of Heaven, Great Wall at Mutianyu section.

The best time to visit the Temple of Heaven is in the morning. Many locals do morning exercise there, taiji, singing, dancing...

Most of them occupy large area. You will walk a lot.